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

The directory where the webapp war should be unpacked is currently set to /var/tmp.  This location, however, is susceptible on some systems to periodic deletion, in which case the web application fails to be found by the server.

For this version, we have simply switched this location to /var/lib/dcache/httpd.

Separate patch for 2.6 will be posted.

Testing:

Built rpm and ran instance, checking for presence of webapp directory in /var/lib/dcache/httpd, and for sanity of webadmin pages.

Target: master
Request: 2.7
Patch: http://rb.dcache.org/r/6002
Require-book: no
Require-notes: yes
Bug: http://rt.dcache.org/Ticket/Display.html?id=7979
Acked-by: Paul

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.