MySQL connect: Parse the Unix domain socket path correctly

MySQL connect: Parse the Unix domain socket path correctly

Some Unix domain socket paths contain colons, requiring MediaWiki to use a modified parsing delimiter. For example,
Google AppEngine provides a /cloudsql/ path to a Unix domain socket used for accessing a Cloud SQL database.
See https://cloud.google.com/sql/docs/mysql/connect-app-engine
The connection name takes the form of project:region:database which introduces two additional colons to the socket path.
Previously, the code presumed the existence of only one colon in a valid Unix domain socket path.

Bug: T230306
Change-Id: I2411ac1b583fd5c143563cb3155ef4e8a29a9b08