Align CONFGROUPS with other options like SUBDIRS and FILESGROUPS by
supporting the CONFGROUPS.yes option. This means logic like this:
.if ${MK_FOO} != "no"
CONFGROUPS+= FOO
.endif
can be replaced by:
CONFGROUPS.${MK_FOO}+= FOO
Use this to simplify libexec/rc/rc.d/Makefile, and while here, indent
and alphabetise the file.
MFC after: 1 week