apropos/whatis: use output of manpath(1) to set defpaths if -M is not
specified. This fixes searching the paths specified in
/usr/local/etc/man.d/*.conf, as currently apropos/whatis from mandoc
suite aren't aware about them.

PR: 227922
Reviewed by: bapt
Approved by: re (gjb), kib (mentor)
Differential Revision: https://reviews.freebsd.org/D17454