SelectDB: Support when database argument is array

When using mysql statements like mysql_select_db($cfg["mysqldb"]);
the tool produced invalid php syntax:
((bool)mysqli_query($GLOBALS["___mysqli_ston"], "USE $cfg["mysqldb"]"));
resulting in
PHP Parse error:  syntax error, unexpected '"', expecting T_STRING or T_VARIABLE or T_NUM_STRING

After this patch, the result is
((bool)mysqli_query($GLOBALS["___mysqli_ston"], "USE " . $cfg["mysqldb"]));

A test should be added, but I am not skilled enough to add one :(

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