snet.UDPAddrFromString resolves hostnames

[matzf]

After #3650, snet.UDPAddrFromString resolves hostnames in the host part of SCION addresses.
This is at least unexpected.

One example can be found in the testcases:

scion/go/lib/snet/udpaddr_test.go

Line 135 in ce1b818

{address: "1-ff00:0:300,[abc]:12", isError: true},

This test fails for us, as there happens to be a host abc in our network and so this call does not return the expected error.

The cause of this issue is that snet.UDPAddrFromString now uses net.ResolveUDPAddr, which resolves hostnames as documented (emphasis mine):

If the host in the address parameter is not a literal IP address or the port is not a literal port number, ResolveUDPAddr resolves the address to an address of UDP end point. Otherwise, it parses the address as a pair of literal IP address and port number. The address parameter can use a host name, but this is not recommended, because it will return at most one of the host name's IP addresses.