Stop talking about the "XDG" version of basename()

Stop talking about the "XDG" version of basename()

XDG refers to X Desktop Group, a former name for freedesktop.org.
This group is responsible for specifications like basedirs,
.desktop files and icon naming, but as far as I know, it has never
tried to redefine basename().

I think these references were meant to say XPG (X/Open Portability
Guide), a precursor of POSIX. POSIX is better-known and less easily
confused with XDG, and is how the basename(3) man page describes
the libgen.h version of basename().

The other version of basename() is glibc-specific and is described
in basename(3) as "the GNU version"; specifically mention that
version, to disambiguate.