Make MySQLi work with non-standard socket

MySQLi needs to specify the socket location as a parameter, and not as part
of the hostname.

Split the socket out of the hostname if it contains *one* colon (IPv6 safe)
after checking for the port number.

Conflicts:
	includes/db/DatabaseMysqli.php

Bug: 65998
Change-Id: I66ee34e1029bc1630669bde4272d0eae9d3fd9f1