CreateNLSocket and CloseNLSocket should return gpointer (mono/mono#15408)

CreateNLSocket and CloseNLSocket should return gpointer (mono/mono#15408)

In managed, these functions are used as `static external IntPtr`. This means that previously on arm64 the top bits of the return value were garbage. A comparison with 64-bit -1 like in LinuxNetworkChange.EnsureSocket would never be true, which was causing us to hit other assertions in the runtime.
Co-authored-by: Aleksey Kliger <alklig@microsoft.com>
Commit migrated from https://github.com/mono/mono/commit/19bb89805ac2eb56b09fed40cab1dd342f97a5e3