Change ADOdb fetch mode to ADODB_FETCH_NUM for Oracle

We were previously using ADODB_FETCH_BOTH, which is less efficient as
each field is returned twice in the recordset. Using ADODB_FETCH_NUM
reduces the memory footprint.

This is a workaround for issue #15426, allowing MantisBT to function
until ADOdb library is patched to fix the problem's root cause.
Install: always report a warning for a blank admin password

Use the DB password as admin password
Oracle: error handing in install when db already exists

Oracle returns error ORA-01920 if attempting to create a user (schema)
which already exists.
Oracle: close and unset ADOdb object after admin connection

This frees up memory, and ensures that the next connection attempts is
done from a clean slate, because some object properties are not reset to
their original values when closing the connection.
Install: Move DB hostname check after DB type

This is more logical and improves code readability
Oracle: DB version information split on 2 lines

The version info contains a newline char, so we use nl2br() t split the
line to improve readability
Install: regroup the form variables initialization
Install: Code cleanup, remove useless if statements

It's not necessary to individually test the same variable to
conditionally display install-only fields
Install: fix problem with db_password when retrying after failure

The logic to avoid divulgation of db password was flawed, forcing user
to re-enter the password when retrying after a failed install/upgrade.
Install: fix logic to retry a failed install

Instead of having a single form spanning the entire page, we have one
to display input fields for user to submit new or modified data, and one
with hidden fields to retry failed steps.

With this, the retry button now works as expected.
WIP: todo - protect admin password