afp: Try to prevent idle disconnects

Some servers (e.g. OS X 10.8) disconnect clients who don't respond to
server notifications.  Prevent this by sending FPGetVolParms, as the
spec suggests.

https://bugzilla.gnome.org/show_bug.cgi?id=734194
afp: Don't retry read if connection closes

Don't retry the read if the remote host closes the connection.  Instead,
exit (semi-)gracefully with exit(0) and exit with an error if there was
an actual error.

https://bugzilla.gnome.org/show_bug.cgi?id=710490
mtp: Fix two harmless warnings

Fix two warnings introduced by be000ea91e90 ("MTP: Emit delete events
when unmounting.").