feature: support non-standard unix socket (#5724)

* Add support for custom MySQL unix-socket

* NULL must be lowercase!

* Naive edit of html/install.php

* fixup

* Refactor dbConnect
Use it everywhere

* $config needs to be global
Don't need to set $database_link

* small cleanups

discovery-wrapper.py

@@ -92,6 +92,9 @@ db_port = int(config['db_port'])
 if config['db_host'][:5].lower() == 'unix:':
     db_server = config['db_host']
     db_port = 0
+elif config['db_socket']:
+    db_server = config['db_socket']
+    db_port = 0
 else:
     db_server = config['db_host']