Improved logging for sync_inv; stricter syntax checking for metafiles
Stored chunks again use own digest

Reverted the change that made stored chunks use the digest of the underlying
pchunk as their own digest, as this, if one changes encryption settings
after uploading backups, could have led to chunks being overwritten in the
target.
Bugfixes related to sync_inv

One strange issue is that the inventory stores the full length of a
composite chunk, which is never listed in a metafile.  In this case, we now
use the length reported by the target, and make sure that it is large enough
to contain the range specified for the component chunk.
Config entry for number of GPG daemons

Added new target config entry to specify the maximum number of child
processes used to encrypt chunks.  In GPGProcManager, child processes are
correctly waited for before new ones can be started.
Improved debug lines