Mk/Uses: Add guile.mk

This patch adds USES=guile[:options], a framework to permit the
concurrent installation of different guile versions, allowing
ports to specify which guile they need.

lang/guile is now a meta-port, installing the default guile version;
guile 3 is now shipped in lang/guile3.

A new port lang/guile-aclocal holds the guile.m4 file from guile3
to permit ports written against guile1 or guile2 to avoid conflicts.

PR:		260960
Reported by:	Martin Neubauer <m.ne@gmx.net>
Approved by:	bofh
Differential Revision:	https://reviews.freebsd.org/D40194