Update db_get_table() calls for table 'user_pref'

Commit ac49ad898958dd72f36fddee72b4b92ef6039d1b adds support for the new
table name syntax,allowing us to replace db_get_table calls within queries.

Calls to db_get_table that are used by other database function,
i.e. db_insert_id, db_field_exists are deliberately left unchanged at this
stage.

This is aimed at breaking down the future DB API changes into manageable
chunks, and to aid the review process after 1.3 for 2.0.

This Pull request deals with calls for the 'user_pref' table only,
for ease of review, and syncing until merged.