Previous refacoring broke all seek/tell within fs. fs handles now lost
the responsibility to keep track of current position. It's blkdev's job.

I'm about to change that structure and I don't want fs to be messed up
by this. I'm proceeding step by step...

The pgm module implements a shell hook so that when an unknown command
is typed, we look into the mounted filesystem and look for a file with
the same name as the command. If we find one, we load it in memory and
run it.

This also introduces the "seek" shell command.

going to use `seek` for block devices