man(1): support spaces in filenames

The globbing function in exists() needs to handle white spaces.

PR: 275978
Reviewed by: kevans, bapt, emaste
Differential Revision: https://reviews.freebsd.org/D43222
MFC after: 1 week

(cherry picked from commit e3c7b76fa8105929ae9a785e5ffc44274b1b0a81)
(cherry picked from commit 7169c947ab4150a539f2af6c9dcf3ab4344b0033)