Implement a RRDP_CLEAR message that instructs the parent to cleanup

Implement a RRDP_CLEAR message that instructs the parent to cleanup
the rrdp directory. This is used before a snapshot download to ensure
that the snapshot is applied to a clean repo.
Similar cleanup happens if the transfer fails. In that case remove the
temp directory contents only.
This uses a new function remove_contents() to remove everything below
a base directory (a bit like rm -r X/*).
OK tb@