Set date to NULL when year is 0000

And remove other now useles checks
Rename reject-root-path to root-dir

Prepare the variable to be a parameter
Add new command line parameter for root-dir

This variable replaces reject-root-path and is used to set the root working
directory.

It defaults to /tmp/pgloader/ like previously.

Also set the logfile according to the root-dir.

TODO: tmpdir is not handled in comand-line. Is it really wanted to have more
command line parameters ?
A word was missing in comment
Review the --root-dir patches for better default management.

Also ensure the directory we're given actually exists on disk, creating it
if necessary, and bail out early in case for whatever reason it's not
possible to create the directory.
Review some log messages and levels.