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.