MySQL Storage Engine support

This big collection of changes does the following:
- Use InnoDB (or, for log_floodcontrol and log_online, MEMORY) by default for new installs (if InnoDB is available - otherwise falls back to MyISAM)
- Add support for specifying a storage engine with $smcFunc['db_create_table']
- Add support for Fulltext indexes for InnoDB tables when running MySQL 5.6.4 or newer
- Add storage engine to array returned by smf_db_table_structure (currently not used in SMF, but some may find it useful)
- Minor code cleanup here and there

Note: In order to allow log_online to be MEMORY, the "url" column in this table was changed from text to varchar(1024). For consistency, I also made this change in PostgreSQL even though there is no benefit for doing so.