MFC: r294362, r294414, r294753

MFC: r294362, r294414, r294753

- Fix tty_drain() and, thus, TIOCDRAIN of the current tty(4) incarnation
  to actually wait until the TX FIFOs of UARTs have be drained before
  returning. This is done by bringing the equivalent of the TS_BUSY flag
  found in the previous implementation back in an ABI-preserving way.
  Reported and tested by: Patrick Powell
- Make the code consistent with itself style-wise and bring it closer
  to style(9).
- Mark unused arguments as such.
- Make the ttystates table const.

git-svn-id: svn://svn.freebsd.org/base/stable/10@294959 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f