Set full-path of cvsup.

In src/Makefile rev 1.232, environment variable PATH is set explicitly
to /sbin:/bin:/usr/sbin:/usr/bin.  As a result, binaries located on
non-standard path cannot be executed without full-path (it's the change
of this revision).

However, cvsup is not in our base system -- you lose if you try to
"make update" without setting SUP in make.conf or command line argument.
I think it is safe to assume that cvsup is located at /usr/local/bin,
and it would help other people who first try to do "make update".

PR:		31932
MFC after:	1 day

Makefile.inc1

@@ -113,7 +113,7 @@ CLEANDIR=	clean cleandepend
 CLEANDIR=	cleandir
 .endif
 
-SUP?=		cvsup
+SUP?=		/usr/local/bin/cvsup
 SUPFLAGS?=	-g -L 2 -P -
 .if defined(SUPHOST)
 SUPFLAGS+=	-h ${SUPHOST}