Rewrite large parts of persistent dirstack handling

Rewrite large parts of persistent dirstack handling

In order to allow filtering what gets committed to the persistent
dirstack, I ended up rewriting the handling altogether.

In order to keep a filtered dirstack, we can't actually derive what gets
saved from $dirstack, unless we'd like to apply the filter to all
entries of the stack each time we're about to save the data to disk.

This uses a new parameter $GRML_PERSISTENT_DIRSTACK, which is our
filtered representation of the actual dirstack.

This code allows for using the same filters after *reading* the saved
dirstack from file while the setup is loading (don't worry, a manual
update with examples is coming up).

On the plus side, this removes the dreaded uprint utility, that we ONLY
used in chpwd when saving dirstack to a file. With this new code, the
GRML_PERSISTENT_DIRSTACK parameter use the -U flag of typeset, which
keeps it unique the whole time.