mancheck: exclude dotfiles in man dir

Its not uncommon for an editor to drop a hidden swap file in the dir
while editing a file there. mancheck would find it and run mandoc on it,
which would complain about its distinctly not-manpage format.

A more correct solution might be to reconfigure the editor to not put
swap files in the same dir, but its the default a lot of the time, and
this is a very small change that gives a very nice quality-of-life
improvement.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #14549