Log Message:
Initial addition of shtk 1.0.

Despite its pretentious name, this package is just an attempt to generalize
a bunch of code that I keep reusing every time I implement shell scripts.
In particular, this is about to remove tons of duplicate stuff from both
sysbuild and sysupgrade in subsequent commits and I'll probably use it again
for some other stuff I have in mind.

Description follows:

The Shell Toolkit, or shtk for short, is a collection of modules written in
sh(1) that provide common functionality to simplify the implementation of
complex shell scripts.  These modules provide things like utilities to
manipulate data types, helpers to expose a common CLI, or higher-level
abstractions such as the processing of configuration files.

The included shtk(1) utility exposes convenience functionality to let the
user "build" shell scripts that use shtk.  Build, in this case, just means
adding common boilerplate code to the initialization of the script to load
the shtk common code.