Always clear serial control lines when used for PTT

When a serial  control line (DTR or  RTS) is used for  PTT control the
rig_open()  function clears  the PTT  control line  after opening  the
port. This  is necessary on Linux  because opening the port  sets them
both as part  of the normal RS-232 protocol. The  lines were not being
cleared when the PTT port was the same port as the CAT control port.
Disable AI mode on open for Kenwood/Elecraft rigs
Add more diagnostics and error checks to serial line control
Move serial line control error checks to lower level routines

Moved in preference to introducing errno.h into rig.c.
Merge branch 'master' into kenwood-ai-off-at-start
Merge branch 'master' into ptt-control-on-cat-port
Merge branch 'kenwood-ai-off-at-start'

of git://git.code.sf.net/u/bsomervi/hamlib