[libc,cmds] Add arena numbers to meminfo -m, prohibit fprintf to fputs conversions

[ghaerr]

While cleaning up meminfo -m blank lines and adding arena numbers instead (see screenshot) it was noticed that meminfo was ~30 bytes over 4K. Trying to save those bytes involved another couple of changes; sharing some strings, and using fd as a global, rather than being passed around to most all the internal functions. Discussed with @Mellvik in Mellvik/TLVC#105 (comment). Sorry about all the new changes to reduce size, just after you ported the previous version, with all its changes!!

It was also noticed that while the -fno-builtin-printf option had already been added, prohibiting GCC compilation conversion of printf to fputs, this did not apply to fprintf being converted to fputs! After reading everywhere for a solution to no avail, I guessed at -fno-builtin-fprintf which was accepted. Recompiling all applications resulted in saving 4K floppy space, apparently from not dragging in the extra fputs code into a number of applications.

Here's meminfo -m without blank lines: