contrib: add an Archlinux makepkg file

makepkg -f PKGBUILD
pacman -U f5gs*.pkg.tar.xz

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
few more fixes to systemd service file

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
fix signaling return value errors

Earlier if permission to send signal was denied the program ran post
signaling script, and passed it's error message to err().  That was both
misleading and wrong also in sense the post executable return value was
not even checked properly.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>