Minor store FS API change

The store FS API dump function has changed syntax and use slighly.

It now is only responsible for printing the FS specific options. The
general syntax and global options is managed at a higher level.

The cache_dir option processing has also been extended with per-option
dump functions, making it a lot easier to remeber all config dump
functions when implementing a store FS.

This change got triggered by segfault on mgr:dump if the "null" store is
used, only because "null" did not implement a config dump function (and
should not need to either, it has no FS specific data).

--
Henrik