srm,dcap: Use configured vomsdir and capath

The code for extracting vomsdir and the capath from certificates ignored the
configured paths. The code also created internal refresh threads with the wrong
diagnostic context.

Introduces srm.authn.vomsdir.

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.10
Request: 2.9
(cherry picked from commit 7f39db9755cd151cbce2ac38f6ecdea3df5c40d3)
Merge pull request #708 from gbehrmann/fix/2.10/rb6904

srm,dcap: Use configured vomsdir and capath