serial gdb: interrupts were causing either overruns or stealing
characters. Put splhigh() around the routines which transfer packets
across the line. Since this happens when the system is halted in
debug, this doesn't cause any particular problem. Now it is possible
to run the link at 115,200 bps.
PR: (not assigned yet, must be in limbo somewhere)
Add partial support for detecting non-existent gdb devices.
Add $FreeBSD$ tag.
097f16c8b7