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)