man(1): Add full search (-K) flag

This flag allows a full text search on man pages. Although this is a last resort
option, it can be useful to pin point a certain man page.

It can be used with -S to narrow the search.

Unlike the Linux version, the search takes place in the rendered text so it
avoids false-positives when the text is found in comments in the source files.

It relies on grep(1) and mandoc(1) to do its job.

Add flag documentation and EXAMPLES to the manual page (bump .Dd).

Usage example:

man -w -K '\<arm\>' -S 1:8

Reviewed By: ceri, emaste, pauamma_gundo.com
Approved by: manpages (bcr@), debdrup@
Differential Revision: https://reviews.freebsd.org/D30984