man/ : More manpage tweaks.

* EXIT STATUS for all
* Mention that the output gets overwritten without asking
* Remove some whitespace mandoc complains about

Patch-from: Jan Stary <hans@stare.cz>
programs/sndfile-metadata-get.c : Make --all the default behaviour.

If given a file name without any options, just behave as if it had
been given the --all option.

Suggested-by: Jan Stary <hans@stare.cz>