This appears to fix the "lots of user.dat files open concurrently" behavior
that remains when a browser/client has the webv4 page open for a long
duration.

Eventually, the User objects created in this loop would be cleaned (and the
user.dat file descriptors closed), but the default garbage collection interval
(configurable) is 1000 calls to the CommonOperationCallback function. This could
be as as long as 1000 times through this loop before GC occurs.

Since this is not a performance sensitive loop (we have a call to sleep for a
full second) just force a garbage collection for each loop iteration.

Probably resolves the "Invalid page" problem reported by W6RAY.

The system.check_name() check *also* verifies that the username is not
already taken, so we must perform the matchuser() check first in order
to get the appropriate error message here when trying to use an alias
(username) that's already taken.

Files were being downloaded as undefined.undefined because I didn't emulate
the old 'base' and 'ext' properties - just use a single 'name' property now.

Synchronet v3.19 now converts ANSI-encoded DIZs to Ctrl-A format upon import.

If brand_link specified in modopts, use that as href for navbar-brand link (BBS name at top left); for CG.

If 'code' query param exists, is a valid xtrn code, and user can access, then jump right into it after the page loads. (Requested by Mortifis.)

SBBS v3.19 introduces a built-in FileBase class. To resolve this
conflict, rename the soon-to-be-deprecated FileBase object to
OldFileBase.

- Default gender of ' ' (instead of '')
- Don't write the gender to the user's birthdate

Exit immediately if client attempting redirect to arbitrary external site. (This won't work for them anyway, but no need to run the rest of the script.)
Use js.exec() for components instead of load().
Use <? ... ?> instead of <?xjs ... ?> for XJS one-liners.

listSubs doesn't need to return that much detail. New stuff will be exported as part of 'forum' object, for use with require().