parse: Zero-out command argument if argument is optional

If a command does not have a corresponding argument, then set the
value to zero rather than re-using the previous value.
gpio-kmem: Add gpio_un/export functions

These are necessary functions for monitoring filehandles for GPIO
edges.  Implement them for the kmem drop-in replacements.
packet: Update FIFO frequency to 130 MHz
main: Get compiling under POSIX
Merge branch 'master' of github.com:xobs/spicode