In the deserialise function, GUnixFDMessage was comparing 'level' to
both SOL_SOCKET and SCM_RIGHTS.  It is correct to compare 'type' to
SCM_RIGHTS.  The code passed tests only because:

1) it's a "should always be OK" double-check

2) SOL_SOCKET and SCM_RIGHTS, by chance, both have the value '1' on
   Linux systems.