Some of these appear to not be in POSIX:
functions: setfsuid, setresuid, setfsgid, setresgid.
errno constants: check
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/errno.h.html
signal constants: check
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html
open() flags: check
https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html
They are probably specific to other operating systems. If they are really
required, we can look into how to implement them in Haiku.
This leaves us with a shorter list of functions from POSIX actually not in
Haiku:
2. Functions
fdatasync
mlockall
munlockall
tcgetsid
S_IFWHT
MCL_CURRENT
MCL_FUTURE
F_GETOWN
F_SETOWN
SOCK_RDM
