Small renames to make filenames more intuitive or easier to deploy.


Note: This introduces a few backward-incompatible names! Be careful while
updating.

* parse_files.inc.php, check_servers.sh: Plaintext log files that will be
served via Apache now have .txt suffix.

* parse_files.inc.php, check_servers.sh: Renamed last_log to
last_global_log, last_success to last_success_date, last_test to
last_test_date.
Documenting what are the valid chars for the id column.