docs: update FULL_POSIX_AUDIT.md sections 2B/2C/2D — mark 14 headers, 20+ functions...

docs: update FULL_POSIX_AUDIT.md sections 2B/2C/2D — mark 14 headers, 20+ functions, 9 errno codes as implemented

Many items listed as missing were already implemented since the original audit.
- Section 2B: 14/27 headers now exist (setjmp, locale, pwd, grp, regex, fnmatch, getopt, libgen, select, resource, utsname, poll, wait, inttypes)
- Section 2C: stdio (perror, popen, fdopen, fileno, getline, ungetc, clearerr), stdlib (strtoul, strtoll/strtoull, atexit, setenv, rand, abort), string (strerror, strtok_r, strpbrk, strspn/strcspn, strnlen), unistd (sleep, execvp/execlp/execl, pipe2, gethostname, ttyname, sysconf/pathconf), signal (sigset_t macros, signal())
- Section 2D: /etc/hosts, FIONREAD, setenv/putenv, process groups, permissions, free()
- errno: 9/11 codes exist (ERANGE, ECONNREFUSED, ETIMEDOUT, EADDRINUSE, EOVERFLOW, ELOOP, ENAMETOOLONG, ESPIPE, EROFS)