improved documentation for atime.gap

The old documentation for atime.gap seemed to be not very exact and a bit
misleading, especially it lacked information about absolute differences being
used.

* Rewrote the documentation for atime.gap based on the code in modules/dcache-chimera/src/main/java/org/dcache/chimera/namespace/ChimeraNameSpaceProvider.java
  including an example for how it works.
use global config options

* Use the global configuration options for the pathnames of the grid host
  certificate and the key.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
corrected small typo in X.509

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Merge pull request #36 from calestyo/atime.gap-documentation

improved documentation for atime.gap
Merge pull request #40 from calestyo/missing-files-semsg-cert-locations

Missing files semsg cert locations.

Signed-off-by: Christoph Anton Mitterer <christoph.anton.mitterer@lmu.de>
Acked-by: Gerd Behrmann <behrmann@nordu.net>