pkg: use specific CONFSNAME_${file} for FreeBSD.conf

Setting CONFSNAME directly is a little more complicated for downstream
consumers, as any additional CONFS that are added here will inherit the
group name by default.  This is perhaps arguably a design flaw in CONFS
because inheriting NAME will never give a good result when additional
files are added, but this is a low-effort change.

While we're here, pull FreeBSD.conf.${branch} out into a PKGCONF
variable so one can just drop a new repo config in entirely with a new
naming scheme. CONFSNAME gets set based on chopping anything off after
".conf", so that, e.g.:

- FooBSD.conf => FooBSD.conf
- FooBSD.conf.internal => FooBSD.conf

Reviewed by:	bapt, manu
Differential Revision:	https://reviews.freebsd.org/D28767

usr.sbin/pkg/Makefile

@@ -15,8 +15,9 @@ PKGCONFBRANCH?=	latest
 .  endif
 . endif
 .endif
-CONFS=		FreeBSD.conf.${PKGCONFBRANCH}
+PKGCONF?=	FreeBSD.conf.${PKGCONFBRANCH}
-CONFSNAME=	FreeBSD.conf
+CONFS=		${PKGCONF}
+CONFSNAME_${PKGCONF}=	${PKGCONF:C/\.conf.+$/.conf/}
 CONFSDIR=	/etc/pkg
 CONFSMODE=	644
 PROG=	pkg