MFC r294235:

MFC r294235:

  Make PPS ASSERT/CLEAR events match the RS-232 signal levels as per RFC 2783.
  Previously the polarity was for TTL levels, which are the reverse of RS-232.

  Also add handling of the UART_PPS_INVERT_PULSE option bit in the sysctl
  value, the same as was recently added to uart(4), so that people using TTL
  level connections can request a logical inverting of the signal.

  Use the named constants from the new dev/uart/uart_ppstypes.h for the pps
  capture modes and option bits.