Make db_get_table() backwards-compatible with 1.2

Issue 0010914 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 0016038