libflux: refactor reactor/watcher implementation

Problem: the reactor and watcher implementations are unnecessarily
exposed and co-mingled.

Split up reactor.c, reactor.h (public), reactor_private.h into:

reactor.c, reactor_private.h
  Only the reactor struct and implementation.
  Add an accessor for the ev_loop, to be used only in watcher_wrap.c

watcher.c, watcher_private.h
  Only the watcher struct and generic "class" implementation.
  Add accessors to avoid exposing watcher struct details to watcher
  implementations.

watcher_wrap.c
  Wrapped libev watcher implementations.
  These now use watcher accessors rather than directly accessing
  the watcher struct.

A public watcher.h header is added to split out the public watcher
interfaces from reactor.h.  This was not strictly necessary, but
reactor.h was a little busy.  Users are still expected to just include
<flux/core.h>.

Update the zmq watcher, flux_t handle watcher, fbuf watchers, and sdbus
watcher to include watcher_private.h and use watcher accessors.

Replacing the inlines in reactor_private.h with actual functions
in watcher.c necessitated linking some executables with
src/common/libflux/watcher.lo.

libev use is now localized to reactor.c and watcher_wrap.c, which should
make transitioning to a new implementation somewhat easier than before.
The code should also be easier to read and navigate.

src/cmd/Makefile.am

@@ -175,6 +175,7 @@ flux_start_LDADD = \
 
 flux_job_LDADD = \
 	$(top_builddir)/src/common/libsubprocess/libsubprocess.la \
+	$(top_builddir)/src/common/libflux/watcher.lo \
 	$(top_builddir)/src/shell/libmpir.la \
 	$(top_builddir)/src/common/libjob/libjob.la \
 	$(top_builddir)/src/common/libutil/libutil.la \
@@ -185,6 +186,7 @@ flux_job_LDADD = \
 
 flux_exec_LDADD = \
 	$(top_builddir)/src/common/libsubprocess/libsubprocess.la \
+	$(top_builddir)/src/common/libflux/watcher.lo \
 	$(fluxcmd_ldadd)
 
 flux_terminus_LDADD = \

src/common/libflux/Makefile.am

@@ -25,6 +25,7 @@ fluxcoreinclude_HEADERS = \
 	handle.h \
 	connector.h \
 	reactor.h \
+	watcher.h \
 	msg_handler.h \
 	message.h \
 	msglist.h \
@@ -62,6 +63,9 @@ libflux_la_SOURCES = \
 	connector_local.c \
 	reactor.c \
 	reactor_private.h \
+	watcher.c \
+	watcher_private.h \
+	watcher_wrap.c \
 	hwatcher.c \
 	msg_handler.c \
 	message.c \

src/common/libflux/flux.h

@@ -15,6 +15,7 @@
 #include "message.h"
 #include "handle.h"
 #include "reactor.h"
+#include "watcher.h"
 #include "msg_handler.h"
 #include "connector.h"
 #include "msglist.h"

src/common/libflux/hwatcher.c

@@ -17,7 +17,7 @@
 
 #include "src/common/libutil/errno_safe.h"
 
-#include "reactor_private.h"
+#include "watcher_private.h"
 
 struct hwatcher {
     flux_watcher_t *fd_w;