MFC r286469, r286591, r286595, r286596, r286613:

MFC r286469, r286591, r286595, r286596, r286613:

  Provide the tty-layer mutex when initializing the pps api.  This allows
  time_pps_fetch() to be used in blocking mode.

  Allow the choice of PPS signal captured by uart(4) to be runtime-configured,
  eliminating the need to build a custom kernel to use the CTS signal.

  Correct the polarity of the PPS assert and clear events with respect to the
  electrical signals on the serial port.

  Document the change in polarity of the uart(4) PPS capture.

  Style fix -- do the braces for switches correctly.

Relnotes:	yes

git-svn-id: svn://svn.freebsd.org/base/stable/10@287037 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f