[kernel][libc][cmds] Add devname libc function

[ghaerr]

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[ghaerr]

Adds linuxmt/devnum.h as kernel header file for macros to access various block devices, in preparation for possible change of minor device numbers in the bioshd driver. Removes all hardcoded numbers from kernel source.

Adds devname to C library. By default opens /dev and reads directory for dynamic lookup of device names. The old versions contained in df.c and mount.c used hard-coded major/minor device numbers. This can be re-enabled for very slow floppy-only system using USE_FASTVERSION=1 in libc/misc/devname.c.

Rewrites ttyname, updates ps to use faster version of internal dev_name using less space.