Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

o Don't make the SER_INT_* defines visible to userland. They
are related to internals, not user-visible state.
o Add a typedef for serdev_intr_t and protect it with !LOCORE.

MFp4:
o Add defines for the 5 interrupt sources typical for serial devices.
These defines can be used for more finegrained interrupt handling
between drivers that cooperatively handle multiple serial ports.
o Add defines for the various bitmasks applicable when all information
is passed between drivers as a single integral.

MFp4:
style(9): <tab> after #define

Make the relationship between modem control bits and their delta cousins
public.

Add a tty-independent include file for definitions pertaining to serial
ports.

Define the most commonly used modem signals, and delta-bit definitions
for them.