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)
