Fix mutex pretty printer test and pretty printer output.

This fixes the mutex pretty printer so that, if the owner ID isn't recorded
(such as in the current lock elision implementation), "Owner ID" will be shown
as "Unknown" instead of 0. It also changes the mutex printer output so that it
says "Acquired" instead of "Locked". The mutex tests are updated accordingly.

In addition, this adds a paragraph to the "Known issues" section of the
printers README explaining that the printer output isn't guaranteed to cover
every detail.

2017-01-14  Martin Galvan  <martingalvan@sourceware.org>

        * README.pretty-printers (Known issues): Warn about printers not
        always covering everything.
        * nptl/nptl-printers.py (MutexPrinter): Change output.
        * nptl/test-mutex-printers.py: Fix test and adapt to changed output.