Fix DB prefix/suffix init for new Mantis installs

When installing a new MantisBt instance, the installer pre-fills the
database prefix, plugin-prefix and suffix inputs.

Due to a bug in the code, all 3 fields were set to the value of
db_table_prefix ('mantis' by default), resulting in the installer
creating tables like 'mantis_bug_mantis` or 'mantis_mantis_xxx_mantis'
instead of 'mantis_bug_table' or 'mantis_plugin_xxx_table' unless the
administrator noticed and fixed the values before launching the install.

Adjust the code that initializes the inputs to use the correct values
(db_table_plugin_prefix, db_table_suffix).

Fixes 0026596