Allow rc_conf_files to be redefined in rc.conf(5)

With this change, it's possible to redefine rc_conf_files (e.g.,
sysrc rc_conf_files+=/etc/rc.conf.other) and have the boot process
pick up settings in extra files. The sysrc(8) tool can be used to
query/enumerate/find/manage extra files configured in this manner.

Relnotes:	yes

etc/defaults/rc.conf

@@ -716,5 +716,17 @@ if [ -z "${source_rc_confs_defined}" ]; then
 				;;
 			esac
 		done
+		for i in ${rc_conf_files}; do
+			case ${sourced_files} in
+			*:$i:*)
+				;;
+			*)
+				sourced_files="${sourced_files}:$i:"
+				if [ -r $i ]; then
+					. $i
+				fi
+				;;
+			esac
+		done
 	}
 fi