GPS app command input

• My main goal for today is to try to get input from both serial ports working. I decided to start by putting the read ports in non-blocking mode and polling - that seems easier than a multi-threaded approach, which might require a whole RTOS or something.
• Gordon is here and is working on the version of his code that computes & checks checksums on all lines, since my data files now have those even on the new custom $PPSCNTR lines.
• At the end of the day, Gordon had almost finished fixing up his code, and I (Mike) was almost finished with the GPS app changes. I got non-blocking fgets() calls activated on the serial ports, but now I am having a problem where the re-entrant versions of the fprintf() and fputs() routines that I use for serial output don't seem to be available. I might have to rewrite the ouput code to use sprintf()+fwrite() or some such.