Add Icom IC-7100 support.

From Martin, CT1IQI:

"Several programs under Linux rely on Hamlib for control. I wanted to
try WSJT-X (digital modes like JT65) and found that my new IC-7100 was
not yet supported, also after having compiled the current git version of
Hamlib and having compiled WSJT-X against that.

So I added a IC-7100 by taking the ic-7200 and 7420 rig files as
example, be it without going (yet) through all of the very many commands
the ic-7100 supports.

This produced the situation where there was communication, e.g. setting
and reading frequencies, but the PTT control did not work.  I debugged
that to actually the lack of a PTT mode in Hamlib that uses serial and
CAT at the same time; currently PTT per 'serial' seems equivalent to
toggling certain RS232 pins but not to any serial command level. So I
added a RIG_PTT_SERIAL_CAT mode for PTT control. Now the wsjt-x program
works nicely with the ic-7100 and controls both frequency and PTT via
the single USB cable."

Signed-off-by: Nate Bargmann <n0nb@n0nb.us>