man: Add -l option

Add a -l option which causes man to interpret all arguments as paths to
open directly rather than man pages to search for in MANPATH. See the
PR for a detailed rationale.

PR: 289245
MFC after: 1 week
Reviewed by: ziaee, emaste
Differential Revision: https://reviews.freebsd.org/D52385

(cherry picked from commit 14b61b2e931741281d0bfef426e9809f16006504)