Add "fc", "fcfontsdir", "fontsdir", and "none" arguments to USES=fonts.
They controls whether fc-cache and/or mkfontdir run in
post-{,de}install targets.

japanese/font-* use manually generates fonts.dir entries to utilize
special modifiers which are useful for CJK fonts. Because mkfontdir
does not understand and silently remove them from fonts.dir,
options like fonts:fc or fonts:none is required.

Approved by: portmgr (bapt)