Move disktab to sbin/bsdlabel/

This leverages CONFS to handle the install.

Approved by:	re (blanket, pkgbase), will (mentor)
Differential Revision:	https://reviews.freebsd.org/D17217

etc/Makefile

@@ -13,7 +13,6 @@ SUBDIR+=sendmail
 .endif
 
 BIN1=	\
-	disktab \
 	group \
 	hosts.allow \
 	libalias.conf \

sbin/bsdlabel/Makefile

@@ -3,6 +3,7 @@
 
 .PATH: ${SRCTOP}/sys/geom
 
+CONFS=	disktab
 PACKAGE=runtime
 PROG=	bsdlabel
 SRCS=	bsdlabel.c geom_bsd_enc.c