GProxyAddressEnumerator: add default-port property

Although none of the in-tree GSocketConnectable types need it, other
types (like SoupAddress) may find it useful to be able to pass a URI
and a default-port to GProxyAddressEnumerator separately (the same way
you can with GNetworkAddress). So add a default-port property.

https://bugzilla.gnome.org/show_bug.cgi?id=698877
Declare res_init() for builds against Android's Bionic

res_init() exists in the C library here and as such is detected
by configure, but it is not declared in any header.