Add PURPLE_MESSAGE_REMOTE_SEND flag to PurpleMessageFlags

Specifies messages like _SEND that were sent from another location, and are
not echoes.

This allows the UI to distinguish them from groupchat outgoing messages, since
those always result in a serv_got_chat_in() call which writes to the
conversation with the PURPLE_MESSAGE_SEND flag, because that's needed to
display outgoing messages in UIs like pidgin, but some UIs behave differently.