Add USES=xorg USES=gl, ports categories m
Add USES=xorg and USES=gl to ports in categories starting with 'm'
While here, try to sprinkle other USES (mostly gnome and sdl) as needed.
Convert most of remaining ports that depend on xorg-libraries to
modular xorg.
- supply corresponding USE_XORG for all imake-using ports that need it
- USE_IMAKE no longer implies USE_XLIB in absence of USE_XORG
- retire USE_X_PREFIX which is not really used anywhere after the
above change
- a few minor nits like whitespace and SF macro
Tested by: 2 tinderbox runs by pav
Approved by: portmgr (pav)
