Add some safeguards.

Add some safeguards against running a Native Backup Job on NDMP client
or storage.

Fixes #329: Add some safeguards against running a Native Backup Job on
            NDMP client or storage.
Don't save filehist for NDMP backup when requested.

Some NDMP implementations send their FILEHIST information in an out of
order way which we currently don't handle fully correct so allow the user
to set a config option to allow not to save the FILEHIST only count the
files and directories saved.