Replace get_server_flag() with get_server_umode()

This function gets the status of a single umode flag identified by umode char rather than
internal server flag number.

This also lets us remove all the USER_MODE* constants because they're no longer required.

Invert the sense of the +w check in e_wall() - we only echo our OPERWALL if we're *not* +w,
because if we are +w then the server will echo it back to us anyway.

include/module.h

@@ -10,7 +10,7 @@
  * if we change the table below, we change this module number to the 
  * current date (YYYYMMDDxx where xx is a serial number).
  */
-#define MODULE_VERSION 2017120601UL
+#define MODULE_VERSION 2017120602UL
 
 #include "struct.h"
 
@@ -285,7 +285,7 @@ enum FUNCTION_VALUE
 	GET_SERVER_MOTD,
 	GET_SERVER_OPERATOR,
 	GET_SERVER_VERSION,
-	GET_SERVER_FLAG,
+	GET_SERVER_UMODE,
 	GET_SERVER_PORT,
 	GET_SERVER_LAG,
 	GET_SERVER2_8,