Support tracking and limiting of user file downloads "per day"

New SCFG->System->Security Options->Security Level Values...
"Downloads Per Day" (D/D): defaults to 0/Unlimited for each level. So by
default, there's no change in behavior for systems.
This feature does *not* supercede any other download restriction (e.g. credit
requirements, file ratios, per-directory download requirements). You can't use
this feature to fully remove a user level's ability to download any files
(i.e. set to 0 downloads per day); use the 'D' or 'T' user restrictions for
that purpose instead. If you disagree with this design, let me know (ASAP).

New ARS Keyword: "DLT" (Downloads Today), this can be used as a download
requirement per file directory or anywhere else ARS keywords are sold (used).
e.g. to limit access/feature to only those that have downloaded 0-99 files
today, use "DLT NOT 100".

"Per day" means since the most recent midnight in the BBS's time zone.

New JavaScript User class properties:
- user.stats.downloads_today
- user.limits.downloads_per_day

New @-code:
- DTODAY (number of files downloaded by user today)

New text.dat string:
- NoMoreDownloads

This implements feature request #938 by Xbit:
"ability to govern number of files downloadable per day"