Add darwin support for remote-client

Add the ability to cross-compile podman remote for OSX.

Also, add image exists and tag to remote-client.

Signed-off-by: baude <bbaude@redhat.com>

cmd/podman/platform_unsupported.go

@@ -0,0 +1,6 @@
+// +build !linux
+
+package main
+
+func CheckForRegistries() {
+}