Add a few checks for (MASTER|PATCH)_SITES groups.

While make(1) is ok with variable names having quite a lot of strange
characters in them, the fetch code mostly uses sh(1), where variable
names can't include a dash (or pluses, or many other things).

PR: 210251 210198
Submitted by: mat
Exp-run by: antoine
Sponsored by: The FreeBSD Foundation, Absolight
Differential Revision: https://reviews.freebsd.org/D6779