Make db_get_table() backwards-compatible with 1.2

Issue #10914 changed the way db_get_table() generates the physical
table name, as well as the function's input parameter ('TABLENAME' vs
'mantis_TABLENAME_table'). This change broke the API, forcing all
plugins to be adapted.

In order to minimize the impact on plugin maintainers, the function
now accepts both the old and the new style parameter.

Fixes #16038
Trigger warning when using old-style db_get_table()

Follow-up commit on fix to issue #16038, a warning is triggered to
inform user that deprecated functionality is being used, encouraging
plugin developers to update their code.
Fix #16039: Login page causes error in web server log

Regression introduced by 564f92a02ff738645d68b6e7f5ace1009b5c5f9f.

Dynamic CSS 'status_config.php' page now exits without generating any
stylesheets when called from login_page.php (status colors are not
needed there anyway).