srm,gplazma: Use CertPath as x509 cert chain credential in Subject

Addresses that two Subjects cannot be compared using its
equals method if the credential sets contain an certificate
chain.

Addresses a problem in the SRM that causes the gPlazma cache
to never have any hits.

The problem is caused by storing the certificate chain as
an array. Since two arrays are never equal according to
their equals methods, two Subjects cannot be equal either
if they contain X.509 certiticate chains. A consequence of
this bug is that the gPlazma cache in the SRM never finds
a hit as Subjects are never equals to those of a previous
request.

The patch solves the problem by using a CertPath instead.
This class is serializable and implements equals and
hashCode methods. JAAS itself uses CertPath as a public
credential in Subjects.

Target: trunk
Request: 2.7
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5941/
(cherry picked from commit bc81889861236c245ff60e0a3fe73d2d25c0c787)
Merge pull request #173 from gbehrmann/fix/2.7/rb5941

srm,gplazma: Use CertPath as x509 cert chain credential in Subject