MFC r267667:
use .Mt to mark up email addresses consistently (part1)

PR: 191174
Submitted by: Franco Fichtner <franco at lastsummer.de>

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Use existing functions mtx_lock() and mtx_unlock().
- Change variable name to 'error', as this is what is mostly used for
functions that return an error.
- Add mutex(9) to the SEE ALSO section.
- Bump the date.

I don't really like the example code. I'd prefer symmetry where possible, eg.

mtx_lock(&example_lock);
error = example(NULL, EXAMPLE_ONE);
mtx_unlock(&example_lock);
if (error != 0)
return (error);

But I'll leave it as it is for now.

Reviewed by: simon

Move the LOCKING section to before the EXAMPLES section.
Make example locking calls in the EXAMPLES section.

Reviewed by: keramida

A few more mdoc fixes I missed in the suggestions of Ruslan.

Add a sample manpage for section 9.

Suggested by: simon
Reviewed by: ru