doc: Update status of functions that are removed from POSIX:2024.
* doc/pastposix-functions/_longjmp.texi: Moved here
fromdoc/posix-functions/.
* doc/pastposix-functions/_setjmp.texi: Likewise.
* doc/pastposix-functions/_tolower.texi: Likewise.
* doc/pastposix-functions/_toupper.texi: Likewise.
* doc/pastposix-functions/fattach.texi: Likewise.
* doc/pastposix-functions/fdetach.texi: Likewise.
* doc/pastposix-functions/ftw.texi: Likewise.
* doc/pastposix-functions/getitimer.texi: Likewise.
* doc/pastposix-functions/getmsg.texi: Likewise.
* doc/pastposix-functions/getpmsg.texi: Likewise.
* doc/pastposix-functions/gets.texi: Likewise.
* doc/pastposix-functions/gettimeofday.texi: Likewise.
* doc/pastposix-functions/ioctl.texi: Likewise.
* doc/pastposix-functions/isascii.texi: Likewise.
* doc/pastposix-functions/isastream.texi: Likewise.
* doc/pastposix-functions/posix_trace_*.texi: Likewise.
* doc/pastposix-functions/pthread_getconcurrency.texi: Likewise.
* doc/pastposix-functions/pthread_setconcurrency.texi: Likewise.
* doc/pastposix-functions/putmsg.texi: Likewise.
* doc/pastposix-functions/putpmsg.texi: Likewise.
* doc/pastposix-functions/setitimer.texi: Likewise.
* doc/pastposix-functions/setpgrp.texi: Likewise.
* doc/pastposix-functions/sighold.texi: Likewise.
* doc/pastposix-functions/sigignore.texi: Likewise.
* doc/pastposix-functions/siginterrupt.texi: Likewise.
* doc/pastposix-functions/sigpause.texi: Likewise.
* doc/pastposix-functions/sigrelse.texi: Likewise.
* doc/pastposix-functions/sigset.texi: Likewise.
* doc/pastposix-functions/tempnam.texi: Likewise.
* doc/pastposix-functions/toascii.texi: Likewise.
* doc/pastposix-functions/ulimit.texi: Likewise.
* doc/pastposix-functions/utime.texi: Likewise.
* doc/gnulib.texi (Function Substitutes): Don't include them here.
(Legacy Function Substitutes): Include them here.