Introduce get_kick_reason() and switch all kick commands to use it

get_reason() is then used only for the implementation of get_kick_reason()
and for the $getreason() scripting function.  Add current nick as an
explicit argument for get_reason(), which pushes all use of from_server
out of the ransom reason functions.

Also switch a few instances of send_to_server() to my_send_to_server() where
the current server has been returned by prepare_command().

source/functions.c

@@ -5081,9 +5081,9 @@ char *blah;
 
 BUILT_IN_FUNCTION(function_getreason, word)
 {
-char *nick = NULL;
+	char *nick = NULL;
 	GET_STR_ARG(nick, word);
-	RETURN_STR(get_reason(nick, word));
+	RETURN_STR(get_reason(nick, get_server_nickname(from_server), word));
 }
 
 BUILT_IN_FUNCTION(function_chmod, words)