Add `drive-dump`.

This quickly dumps the contents of drives to a directory. It copies
files first and makes a full image afterwards.

A link (`dm` -> `drive-dump`) is included as well.
Fix tabs in Makefile.
Update todo.
Make small semantic changes to scripts.

Some 'sh' compatibility has been removed.
Add `git-remote-mod-url`.

This uses sed to apply a pattern to a remote url.