Correctly close subshell.

Blocks in a subshell must be closed before the subshell exits. This can
probably checked on exit from a subshell.  Traps set in the subshell
must be saved and restored then.  How about providing an `on_exit`
function as a generic endpoint for exiting a shell, i.e. just always
`trap "on_exit" EXIT` and define `on_exit` in the application.
Add `shtuc-local-root`
Describe `shtuc-global-root`

This script should output the path to the shtuc directory for all of the
users projects, e.g. either in XDG_CONFIG_HOME or HOME.
Add `shtuc-global-root`
Describe `shtuc-system-root`
Add `shtuc-system-root`