Fix spurious stat-changed signals

We were emitting spurious stat-changed GObject signals on receipt of a
Unix signal, causing the status window to report I/O and the app to
display an "unable to download chunks" error.
Put vmnetfs in its own process group

This ensures that INT/QUIT/TSTP/HUP signals sent from the terminal
won't cause us to stop or terminate uncleanly.

Closes #31.