os: Use LOCAL_PEERCRED instead of getpeereid() on FreeBSD

Inspired by 165d5c12.

LOCAL_PEERCRED (FreeBSD, DragonFly) is similar to SO_PEERCRED (Linux, OpenBSD). getpeereid() uses LOCAL_PEERCRED but getpeerucred() is missing, so there's no wrapper to get cr_pid.

Fixes XResGetClientPid() used by

• https://github.com/resolritter/wpid-by-wid
• https://github.com/Plagman/gamescope/commit/70af2c38f0f1
• https://github.com/seebye/ueberzug/commit/11aad5190316
• https://gitlab.gnome.org/GNOME/libwnck/-/commit/07694559cc0c
• https://github.com/mate-desktop/marco/commit/2540175e5a5b

CC @manu, @arichardson