Remove the 'ether' module dependancy, it is not defined anywhere and will

Remove the 'ether' module dependancy, it is not defined anywhere and will
prevent the loader from pre-loading the modules in question.

Do not partially initialize the related struct sockaddr_in when a
non-existant or "0.0.0.0" pxe related kenv is found.  In particular, if
the gateway is not specified by the DHCP server we avoid attempting to add
a bad default route (which results in a panic) later on.