ftp: Implement delayed passive and enable redirect FTP for IPv6

Implements the Globus delayed passive extension. Delayed passive
allows dCache to redirect passive data channels to pools without
using GridFTP 2 GET and PUT commands. Delayed passive can be used
with IPv6, which is not the case for GET and PUT.

Pools that do not support IPv6 or pools older than 2.9 will fall
back to using the door as a proxy if an IPv6 data channel is
requested.

Target: trunk
Require-notes: yes
Require-book: yes
Patch: http://rb.dcache.org/r/6895/
Patch: http://rb.dcache.org/r/6900/
Patch: http://rb.dcache.org/r/6902/
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>