MDEV-15935 Adding global/session system var redirect_url

Adding a global/session var `redirect_url' of string type. The initial
value is empty. Can be supplied in mysqld with --redirect-url or set
in --init-connect. A valid redirect_url should be of the format

{mysql,mariadb}://host[:port]

where <host> is an arbitrary string not containing colons, and <port>
is a number between 0 and 65535 inclusive.

The variable will be used by the server to notify clients that they
should connect to another server, specified by the value of the
variable, if not empty.

The notification is done by the inclusion of the variable in
session_track_system_variable.

mysql-test/main/mysqld--help.result

@@ -1064,6 +1064,8 @@ The following specify which files/extra groups are read (specified before remain
  --read-rnd-buffer-size=# 
  When reading rows in sorted order after a sort, the rows
  are read through this buffer to avoid a disk seeks
+ --redirect-url=name URL of another server to redirect clients to. Empty
+ string means no redirection
  --relay-log=name    The location and name to use for relay logs.
  --relay-log-index=name 
  The location and name to use for the file that keeps a
@@ -1838,6 +1840,7 @@ read-binlog-speed-limit 0
 read-buffer-size 131072
 read-only FALSE
 read-rnd-buffer-size 262144
+redirect-url 
 relay-log (No default value)
 relay-log-index (No default value)
 relay-log-info-file relay-log.info
@@ -1869,7 +1872,7 @@ secure-timestamp NO
 server-id 1
 session-track-schema TRUE
 session-track-state-change FALSE
-session-track-system-variables autocommit,character_set_client,character_set_connection,character_set_results,time_zone
+session-track-system-variables autocommit,character_set_client,character_set_connection,character_set_results,redirect_url,time_zone
 session-track-transaction-info OFF
 show-slave-auth-info FALSE
 silent-startup FALSE