insert#href: implement additional methods [bug 7179]

Added support for PUT and DELETE to insert#href.
Enable execution of FTP requests in handler threads by -DFTP_USE_HANDLER_THREADS.
The <insert> tag no longer requires source="<plugin>" to allow plugins to override RXML flags.
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 devel

* patches/bug2748:
  FTP: Generate the ls(1) output synchronously in handler mode.
  FTP: Improved command serialization when FTP_USE_HANDLER_THREADS.
FTPS: Fixed support for Pike 8.0.
Merge branch 'patches/pike8.0' into devel

* patches/pike8.0:
  FTPS: Fixed support for Pike 8.0.
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].