Enable execution of FTP requests in handler threads by -DFTP_USE_HANDLER_THREADS.
FTP: Improved command serialization when FTP_USE_HANDLER_THREADS.

Now uses (a slightly modified) Thread.Queue instead of an ADT.Queue.

Make sure that next_cmd()/low_next_cmd() always is executed from the
backend thread.

Both of the above should reduce issues with race conditions.

Fixes some of [bug 2748].
FTP: Generate the ls(1) output synchronously in handler mode.

Fixes some more of [bug 2748] and [bug 2779].
Merge branch 'patches/bug2748' into 5.4

* patches/bug2748:
  FTP: Generate the ls(1) output synchronously in handler mode.
  FTP: Improved command serialization when FTP_USE_HANDLER_THREADS.
Check index files against dir listing before stat.

This can improve performance a whole lot if the underlying filesystem
is slow.
Check banish list before looking up userinfo.

This can improve performance quite a bit if the underlying user DB is slow,
typically with .htaccess lookups. Also added some defaults mentioned in
[Bug 5956].