plugin: let plugins decide if they want echo removal during AT probing

This is the port to git master of the following patch:

commit 21e66dfa1774ac2ee037ac8b6e8bb4d71a6f7931
Author: Dan Williams <dcbw@redhat.com>
Date:   Thu Aug 23 21:13:35 2012 -0500

    core: add function to open probe ports without removing echo

    Some devices (Sierra GSM ones) return stuff we need but don't
    bother to prefix it with <CR><LF>, so we need to optionally turn
    off the echo removal at probe time.

src/mm-plugin.h

@@ -58,6 +58,7 @@
 #define MM_PLUGIN_CUSTOM_INIT               "custom-init"
 #define MM_PLUGIN_CUSTOM_AT_PROBE           "custom-at-probe"
 #define MM_PLUGIN_SEND_DELAY                "send-delay"
+#define MM_PLUGIN_REMOVE_ECHO               "remove-echo"
 
 typedef enum {
     MM_PLUGIN_SUPPORTS_PORT_UNSUPPORTED = 0x0,