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