Fix getnetbyaddr implementation.

There were two problems in the getnetbyaddr implementation.  The type
argument is pretty much useless since (almost) no input file contains
this information and the NSS backends make up the value they fill in
for the n_addrtype field.  Therefore we now declare that passing AF_UNSPEC
is always recognized.  Secondly, the files backend didn't compare the network
numbers with the correct endianess.

Also change getent to take advantage of the type parameter change.

(cherry picked from commit 29ba9812)