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.