devel/universal-ctags: New option GLOBAL_CONFIG

This new option makes Universal Ctags read a global configuration file (at
${LOCALBASE}/etc/uctags.conf) before the user's one.  This is useful in order to
apply some common configuration to all users, who can then tweak whichever part
they want in their own file.

PR:		273645
Approved by:	dereks@lifeofadishwasher.com (maintainer)