Restore the ability to set capenabled directly in syscalls.conf.

This fixes generation of cloudabi syscall tables broken in r340424.

Reviewed by:	kevans, emaste
MFC after:	3 days
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D21821

sys/kern/makesyscalls.sh

@@ -64,7 +64,9 @@ if [ -n "$2" ]; then
 	. "$2"
 fi
 
-if [ -r $capabilities_conf ]; then
+if [ -n "$capenabled" ]; then
+	# do nothing
+elif [ -r $capabilities_conf ]; then
 	capenabled=`egrep -v '^#|^$' $capabilities_conf`
 	capenabled=`echo $capenabled | sed 's/ /,/g'`
 else