Write PID of process to current_mirror file.

---------[ Medium term ]---------------------------------------

Look into sparse file support (requested by Stelios K. Kyriacou)

Look into security.py code, do some sort of security audit.

Look at Kent Borg's suggestion for restore options and digests.

Add --list-files-changed-between or similar option, to list files that
have changed between two times

Add --dry-run option (target for v1.1.x)

Add # of increments option to --remove-older-than

---------[ Long term ]---------------------------------------

Think about adding Gaudet's idea for keeping track of renamed files.

Look into different inode generation techniques (see treescan, Dean
Gaudet's other post).


