[Xastir] GPS NMEA sentences decoded by Xastir

[Menga]

I noted in the file gps.c of the Xastir sources two problems in the decoding 
of GPS NMEA sentences: only two types of sentences are decoded $GPRMC and 
$GPGGA, no checksum is tested.
GPS devices have many different preambles, just in my case $IIRMC and 
$IIGGA, II stands for integrated instruments, but there are many others that 
are banned from Xastir.
Moreover, a control of the checksum is not complicate and it can be added.
I ask to the developers a comment on this issue, and in the case I can offer 
a proposal for modification.
Giuseppe
IZ1GCL/MM