Add comments to demand-loading helper
Add comments to demand-loading helper

There are several reasons:
  - The usage needs to be documented.
  - The configuration file format needs to be documented.
  - The sed(1) statement that strips off comments and empty lines is
    fairly complex.
  - The inner workings are non-trivial.

Later, most of the documentation will be moved into manpages.

Closes: #8

donkarlos/libdl-sh