games/quake2-extras: unbreak the port, including SKINS option

Commit c00c521f6f2f had inadvertently broken some ports, this
included; change DIST_SUBDIR so it remains the same yet passes
!empty(DIST_SUBDIR) check when setting _CKSUMFILES.

Jim Waurzyniak's Quake 2 page previously hosted at University
of Michigan is now defunct, host one of the distfiles locally
while the other one remains to be found.

PR: 240031
Reported by: Alastair Hogge, pkg-fallout