If failing to import file content data, drop contents imported thus far

This patch adds an export+import method to the file command line module. The
file export is agnostic of the file storage backend. It writes the files
with the %file record data to standard-out or to the file given by the -f
option. The file is a considered a stream and can quietly be compressed and
can be read back in as a stream (no seeking is performed in the import
process).

Instead of allowing the backend to send a redirect URL, which doesn't make
sense on the command line

Add the current backend to the list results output
Add a --limit switch to support limit results sent to a command like
`migrate`

Allows for backend listing, file listing, single file dump and file
migration.