Previosly we had a very messed up PS1 as the Shell PROMPT is not
unset correctly.
We now provide a default `zshrc` file for the system that uses
sane values for basic categories like aliases, autocompletion and
history management to make the port more usable. It also forces
the prompt to be the default zsh one.
All of these patches did the same thing, which is already in upstream
config.sub.
With this change, we need only add `use_fresh_config_sub=true` to
the package.sh file.
Note that this is not done automatically in case the port has a modified
config.sub file.
