gplazma: Fix DN extraction from proxy certificates

Addresses an issue in the x509 and xacml plugins in which the DN of
a proxy certificate rather the the DN of the user is extracted.

Extracting the DN of the user owning a proxy certificate involves
traversing the certificate chain, discarding any proxy certificates,
until the user's real certificate is found. The existing code in
dCache is not aware of all possible proxy certificate types, which
in some cases lead it to extract the DN of a proxy certififcate
rather than the user's DN.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/5667/