gmc-mh-1x-2x driver depends on libserialport
Use accessor for new libserialport opaque port structure.
Use new libserialport opaque configuration structure.
serial.c: Re-enable serial_read() error reporting.

libserialport now returns 0 (not SP_ERR_FAIL) as return value when a
non-blocking read would return EAGAIN.

This fixes bug #188.
output/csv: Fix wrong cast.
input/vcd: Coding style fixes.
Don't accept over 64 probes.

The code is limited to 64 probes for now, so don't accept setting
the probe limit to higher than that.

See bug 194.