GSocket: fix g_socket_bind() allow_reuse semantics

GSocket: fix g_socket_bind() allow_reuse semantics

With UDP sockets, g_socket_bind() with allow_reuse=TRUE on Linux
behaved in a way that the documentation didn't suggest, and that
didn't match other OSes. (Specifically, it allowed binding multiple
multicast sockets to the same address.)

Since this behavior is useful, and since allow_reuse didn't have any
other meaning with UDP sockets, update the docs to reflect the Linux
behavior, and make it do the same thing on non-Linux.

https://bugzilla.gnome.org/show_bug.cgi?id=689245