Enable support for p-u packages

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

britney.conf

@@ -3,6 +3,7 @@
 # Paths for control files
 TESTING           = /srv/release.debian.org/britney/var/data-b2/testing
 TPU               = /srv/release.debian.org/britney/var/data-b2/testing-proposed-updates
+PU                = /srv/release.debian.org/britney/var/data-b2/proposed-updates
 UNSTABLE          = /srv/release.debian.org/britney/var/data-b2/unstable
 
 # Output