(2.6) webadmin: eliminate system-specific dir for unpacking of war file

Version of http://rb.dcache.org/r/6002 for 2.6.

See above-cited patch for details.

Target: 2.6
Patch: http://rb.dcache.org/r/6003/
Require-book: no
Require-notes: yes
Bug: http://rt.dcache.org/Ticket/Display.html?id=7979
Acked-by: Paul

Testing:  as with http://rb.dcache.org/r/6002.

RELEASE NOTES:

Fixes bug where on some systems automatic cleanup of temporary directories was eliminating the webapp application files and thus causing the webadmin pages to fail to be found.
Renamed .gitignore to .empty-dir in conformity with dCache usage.
Merge pull request #193 from alrossi/fix/2.6/unpackdir

Fix/2.6/unpackdir