res_pjsip: Add handling for incoming unsolicited MWI NOTIFY

A new endpoint parameter "incoming_mwi_mailbox" allows Asterisk to
receive unsolicited MWI NOTIFY requests and make them available to
other modules via the stasis message bus.

res_pjsip_pubsub has a new handler "pubsub_on_rx_mwi_notify_request"
that parses a simple-message-summary body and, if
endpoint->incoming_mwi_account is set, calls ast_publish_mwi_state
with the voice-message counts from the message.

Change-Id: I08bae3d16e77af48fcccc2c936acce8fc0ef0f3c

main/strings.c

@@ -369,4 +369,23 @@ equals:
 	return 0;
 }
 
+char *ast_read_line_from_buffer(char **buffer)
+{
+	char *start = *buffer;
 
+	if (!buffer || !*buffer || *(*buffer) == '\0') {
+		return NULL;
+	}
+
+	while (*(*buffer) && *(*buffer) != '\n' ) {
+		(*buffer)++;
+	}
+
+	*(*buffer) = '\0';
+	if (*(*buffer - 1) == '\r') {
+		*(*buffer - 1) = '\0';
+	}
+	(*buffer)++;
+
+	return start;
+}