share/mk: support for "single" group options

Support group options where 1 of n values will be selected (or a default
value will be used). After processing, an OPT_FOO will be set to one
value from FOO_OPTIONS for each FOO in SINGLE_OPTIONS. If the user
sets FOO that value will be used, otherwise __FOO_DEFAULT will be used.

Options that don't work an a particular system can be remapped to an
alternative using BROKEN_SINGLE_OPTIONS which can be set to a list of
3-tuples of the form:
OPTION broken_value replacement_value

This is somewhat inspired by OPTIONS_SINGLE from ports, but the
structure is quite different with a per-option variable in the style of
MK_FOO={yes,no}.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D41659