Implement a new set and hierarchy of end-user usage functions:
- engines: prints a list of available procedures
- help: prints the list of procedures together with a brief usage summary
- usage: prints the detailed description for the selected procedure

Function declarations are provided. onlinehelp.cc addresses these new
functions. Function bodies, however, are not yet implemented.

Next:
- implement bodies of usage and classusage functions.
- adjust bodies of implemented classhelp functions.
- new bodies must make use of character strings created automatically
  from *usage.txt files