[Xastir] GPS strings from alternate "talkers"

[Tom Russo]

A long time ago, we serviced an issue request to stop requiring "$GP" at the
beginning of recognized GPS strings, because that restricted use of Xastir
to GPS units that only used one constellation.

The first attempt to fix this changed our code to recognize only the "$G" first
two characters and accept numerous alternate values for the third character.

That enabled Xastir to work with a bunch of newer devices that use additional
satellite constellations like GLONASS, Gallileo, Beidou, etc.

Almost before the electrons were dry on that commit, we got another request
to let Xastir recognize Integrated Instrumentation devices, which have
NMEA sentences starting with $II.

An attempt was made to cram that in at the last minute before our 2.1.2 
release, but there was a mistake in the code that I didn't notice until 
this week, and the original requester never actually got back to us on the
issue saying whether it worked or didn't (it almost certainly didn't).

Does anyone here have a GPS receiver that throws NMEA sentences that do
NOT start with "$G" and don't have P,N,A,L, or B as the third character?  If
so, it would be helpful to get someone to sanity check my latest pull 
request, which I believe actually finishes the issue 121 request correctly.

https://github.com/Xastir/Xastir/issues/121
https://github.com/Xastir/Xastir/pull/183

By "sanity check" I mean "actually build the code as fixed in the pull 
request, and hose it up to a GPS receiver we couldn't work with before to
see if it works now."

-- 
Tom Russo    KM5VY
Tijeras, NM  

 echo "prpv_a'rfg_cnf_har_cvcr" | sed -e 's/_/ /g' | tr [a-m][n-z] [n-z][a-m]