bsd.init.mk: always define _gid when MK_INSTALL_AS_USER is set

A recent commit started to use _uid and _gid in <bsd.dirs.mk> to
mangle the user and group for newly installed directories when
MK_INSTALL_AS_USER is set.  However, _gid was previously only set
when _uid was not 0, causing the group to be set to the empty
string.

Set _uid and _gid together to avoid this problem.

Fixes:		541e6e2d51
PR:		297841
Reviewed by:	emaste, imp
Reported by:	Ralph Zitz <[email protected]>
Differential Revision: https://reviews.freebsd.org/D59150

share/mk/bsd.init.mk

@@ -32,11 +32,11 @@ OBJS_SRCS_FILTER+= R
 _uid!=	id -u
 .export _uid
 .endif
-.if ${_uid} != 0
 .if !defined(_gid)
 _gid!=	id -g
 .export _gid
 .endif
+.if ${_uid} != 0
 .for x in BIN CONF DOC DTB INFO KMOD LIB MAN NLS SHARE
 $xOWN=	${_uid}
 $xGRP=	${_gid}