Convert code in bsd.port.mk for USE_GHOSTSCRIPT* to USES=ghostscript.
Add GHOSTSCRIPT_DEFAULT to bsd.default-versions.mk for easy version selection.
Arguments supported: <empty>, build, run, nox11 and agpl
PR: 201201 (exp-run)
Approved by: portmgr (mat@)
Exp run by: antione@ (previous patch)
Differential Revision: https://reviews.freebsd.org/D2938