chkstow incorrectly assumes /usr/local exists

I'm running Stow on Guix where `/usr/local` does not exist by default. Considering that Stow is now frequently used for managing `.dotfiles`, would it make sense to make the default target directory the users home directory instead? That will probably always exist no matter the distribution. Alternatively, does a default target directory need to be set at all?

Start by reading the chkstow entry point and reproduce the behavior on a system where /usr/local does not exist, such as Guix. Compare the possible outcomes described in the issue—using the home directory or requiring no default target—and define done as a documented, tested decision for the default target behavior.

Written by the indexing model from the issue text.