signals: Updated signal list and defines, introduced RTMAX

1. Removed the intro comment: it's >20 years old, some things clearly no longer hold (UNUSED, the bug in glibc, Linux ports, ...). Replaced it with some boilerplate intro.
2. Updated manual defines of missing macros (explained in comments). Removed the EMT/STKFLT situation (ancient).
3. All functions now account for SIGRTMAX.
4. Placed all known non-RT signals into sigtable[]. There's no need anymore to try hard to only keep uniquely-numbered signals in it and to make them a complete sequence. Most kinds of updates to the signals can be accomplished just by editing the table. All functions accomodate, there's no longer a need for them to maintain exceptions (most of them).
5. No function returns a freeable pointer.
6. strtosig(): Re-write (old one didn't handle synonyms, interpreted malloc() failure as signal not found).
7. On the auxiliary, implemented a stable sort. The stability of qsort() is unspecified.