zless(1): handle uncompressed files

In the manual page for zmore(1) and zless(1) it is said that zless(1) is
equivalent to zmore(1) except that it uses less(1) as a pager. However
zmore(1) is able to handle uncompressed files transparently while zless(1)
is not.

Add another case to the switch in lesspipe.sh to handle non-compressed files.

PR:		196437
Reported by:	marquis@roble.com
Approved by:	mjg@
Differential Revision: https://reviews.freebsd.org/D38674

usr.bin/less/lesspipe.sh

@@ -25,4 +25,6 @@ case "$1" in
 	*.zst)
 		exec zstd -d -q -c "$1"	2>/dev/null
 		;;
+	*)	exec cat "$1"		2>/dev/null
+		;;
 esac