posix: add *at() calls and related constants
posix: add *at() calls and related constants

Add missing calls from the *at() call family that was added in
POSIX.1-2008, and related constants.

Fixes https://gitlab.gnome.org/GNOME/vala/issues/823

GNOME/vala,frida/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala