dcache: httpd service, remove ipv4 specific binding and expose httpd.net.listen

The Jetty ssl connector was hardcoded to listen on "0.0.0.0".  This is an ipv4-specific mask.  The simple connector and the ssl connector also did not allow for a different net binding.

This patch removes the hardcoded binding of host name and exposes an httpd.net.listen property which defaults to dcache.net.listen.

Since new properties cannot be ported to 2.6, a separate patch which simply removes the hard-coded ipv4 setting will be provided.

Testing:

Checked that the default settings still work.

Target: master
Request: 2.6
Patch: http://rb.dcache.org/r/6116
Require-notes: yes
Require-book: no
Acked-by: Gerd

RELEASE NOTES:
Allows for ipv6 binding and (in master) exposes a new property for alternate net binding.