SQL efficiency improvements

Basically this PR removes useless checks which are already done by the SQL database server. I.e. rather than checking if an account exists to either update or insert (which means two lookups on the DB hashmap, one to see if exists, and one to update the rows), it tries to update, and if it fails, then it issues an insert, which means one less SQL query if the row exists.

It also lets users change the column names for the accounts table, so they can upgrade from a different economy plugin without touching the DB at all.

File mode changes just remove the *nix executable flag from source code files, nothing else.