std.os: spring-cleanup and specify organization

- Alphabetically sort things, where reasonable.
- Document, that **only** non-portable posix things belong into posix.zig
  * If there is a portable abstraction, do not offer one in posix.zig
  * Reason: Prevent useless abstractions and needless strong coupling.
- Move posix functions into posix.zig
- Move wasi functions into wasi.zig

Closes ziglang#6600.

lib/std/os.zig

@@ -6,6 +6,10 @@
 //!   for UTF-16LE encoding.
 //! * Where operating systems share APIs, e.g. POSIX, these thin wrappers provide
 //!   cross platform abstracting.
+//!   - If operating systems API semantics can be unified without significant
+//!     drawback, then only the most portable abstraction or wrapper is provided.
+//!     This usually means, that there is no POSIX function, if Windows and POSIX
+//!     API can be used with the same semantics.
 //! * When there exists a corresponding libc function and linking libc, the libc
 //!   implementation is used. Exceptions are made for known buggy areas of libc.
 //!   On Linux libc can be side-stepped by using `std.os.linux` directly.
@@ -15,50 +19,49 @@
 //! in general EINTR is handled by trying again.
 
 const root = @import("root");
-const std = @import("std.zig");
 const builtin = @import("builtin");
+const std = @import("std.zig");
+const dl = @import("dynamic_library.zig");
 const assert = std.debug.assert;
 const math = std.math;
 const mem = std.mem;
 const elf = std.elf;
 const fs = std.fs;
-const dl = @import("dynamic_library.zig");
 const MAX_PATH_BYTES = std.fs.MAX_PATH_BYTES;
 const is_windows = builtin.os.tag == .windows;
 const Allocator = std.mem.Allocator;
 const Preopen = std.fs.wasi.Preopen;
 const PreopenList = std.fs.wasi.PreopenList;
 
 pub const darwin = @import("os/darwin.zig");
+pub const linux = @import("os/linux.zig");
+pub const plan9 = @import("os/plan9.zig");
+pub const uefi = @import("os/uefi.zig");
+pub const wasi = @import("os/wasi.zig");
+pub const windows = @import("os/windows.zig");
+
+pub const posix_spawn = @import("os/posix_spawn.zig"); // posix
+pub const ptrace = @import("os/ptrace.zig"); // posix
 pub const dragonfly = std.c;
 pub const freebsd = std.c;
 pub const haiku = std.c;
 pub const netbsd = std.c;
 pub const openbsd = std.c;
 pub const solaris = std.c;
-pub const linux = @import("os/linux.zig");
-pub const plan9 = @import("os/plan9.zig");
-pub const uefi = @import("os/uefi.zig");
-pub const wasi = @import("os/wasi.zig");
-pub const windows = @import("os/windows.zig");
-pub const posix_spawn = @import("os/posix_spawn.zig");
-pub const ptrace = @import("os/ptrace.zig");
 
 comptime {
     assert(@import("std") == std); // std lib tests require --zig-lib-dir
 }
 
 test {
+    _ = @import("os/test.zig");
     _ = darwin;
     _ = linux;
+    _ = wasi;
+    _ = windows;
     if (builtin.os.tag == .uefi) {
         _ = uefi;
     }
-    _ = wasi;
-    _ = windows;
-    _ = posix_spawn;
-
-    _ = @import("os/test.zig");
 }
 
 /// Applications can override the `system` API layer in their root source file.
@@ -70,8 +73,8 @@ else if (builtin.link_libc or is_windows)
     std.c
 else switch (builtin.os.tag) {
     .linux => linux,
-    .wasi => wasi,
     .uefi => uefi,
+    .wasi => wasi,
     else => struct {},
 };
 
@@ -98,10 +101,10 @@ pub const Kevent = system.Kevent;
 pub const LOCK = system.LOCK;
 pub const MADV = system.MADV;
 pub const MAP = system.MAP;
-pub const MSF = system.MSF;
 pub const MAX_ADDR_LEN = system.MAX_ADDR_LEN;
 pub const MFD = system.MFD;
 pub const MMAP2_UNIT = system.MMAP2_UNIT;
+pub const MSF = system.MSF;
 pub const MSG = system.MSG;
 pub const NAME_MAX = system.NAME_MAX;
 pub const O = switch (builtin.os.tag) {
@@ -122,7 +125,6 @@ pub const RR = system.RR;
 pub const S = system.S;
 pub const SA = system.SA;
 pub const SC = system.SC;
-pub const _SC = system._SC;
 pub const SEEK = system.SEEK;
 pub const SHUT = system.SHUT;
 pub const SIG = system.SIG;
@@ -136,10 +138,11 @@ pub const STDOUT_FILENO = system.STDOUT_FILENO;
 pub const SYS = system.SYS;
 pub const Sigaction = system.Sigaction;
 pub const Stat = system.Stat;
-pub const TCSA = system.TCSA;
 pub const TCP = system.TCP;
+pub const TCSA = system.TCSA;
 pub const VDSO = system.VDSO;
 pub const W = system.W;
+pub const _SC = system._SC;
 pub const addrinfo = system.addrinfo;
 pub const blkcnt_t = system.blkcnt_t;
 pub const blksize_t = system.blksize_t;
@@ -151,6 +154,7 @@ pub const empty_sigset = system.empty_sigset;
 pub const fd_t = system.fd_t;
 pub const fdflags_t = system.fdflags_t;
 pub const fdstat_t = system.fdstat_t;
+pub const file_obj = system.file_obj;
 pub const gid_t = system.gid_t;
 pub const ifreq = system.ifreq;
 pub const ino_t = system.ino_t;
@@ -165,10 +169,9 @@ pub const off_t = system.off_t;
 pub const oflags_t = system.oflags_t;
 pub const pid_t = system.pid_t;
 pub const pollfd = system.pollfd;
-pub const port_t = system.port_t;
 pub const port_event = system.port_event;
 pub const port_notify = system.port_notify;
-pub const file_obj = system.file_obj;
+pub const port_t = system.port_t;
 pub const rights_t = system.rights_t;
 pub const rlim_t = system.rlim_t;
 pub const rlimit = system.rlimit;