[BUGFIX] Replace SHOW DATABASE by query to schema

The SQL query "SHOW DATABASE" may be restricted on the SQL Server.
Since MySQL 5.0 (which is minimum since TYPO3 CMS 4.7) we can also query
the information_schema table to gather the information to which table
we have access.

Resolves: #47744
Releases: 6.2, 6.1
Change-Id: Ie65abadeed54cbc54bee401bdc1aab18ffc8eea6
Reviewed-on: https://review.typo3.org/23795
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters