fix if_nametoindex return value when socket open fails

The return value of if_nametoindex is unsigned; it should return 0
on error.

fix if_nametoindex return value when interface does not exist

the return value is unsigned, so negative results for "errors" do not
make sense; 0 is the value reserved for when the interface name does
not exist.

fix missing SOCK_CLOEXEC in various functions that use sockets internally

implement if_indextoname and if_nametoindex functions