getgroups: document portability issues

getgroups: document portability issues

Because I actually managed to hit deadlock in libvirt's child
process due to glibc's mutex use in user database lookup, I
figured it is worth documenting the issue for others to be
aware of when writing a privileged multithreaded parent app
that spawns child processes owned by non-privileged ids.

* doc/glibc-functions/initgroups.texi (initgroups): Mention
multithread safety.
* doc/posix-functions/getpwuid.texi (getpwuid): Likewise.
* doc/posix-functions/getpwuid_r.texi (getpwuid_r): Likewise.
* doc/glibc-functions/getgrouplist.texi (getgrouplist): Mention
getugroups.
* doc/posix-functions/getgroups.texi (getgroups): Mention
multithread safety and mgetgroups.

Signed-off-by: Eric Blake <eblake@redhat.com>