Find the desired root location when symlinking

When the sub binary is symlink, use the directory where the link is,
rather than the physical path of the file to determine the sub root
directory and the libexec directory.
Allow %COMMAND% placeholder in usage string

Whenever the %COMMAND% string is found in the Usage line of a sub, it
will be replaced by the tokens that lead up to the command to run the
sub.

The goal is to be able to move subs around without having to modify
their source.
Export the command in environment for subs

All subs will get the command used to run them in an environment
variable called ${SUB}_COMMAND, where {SUB} is the name of the sub,
capitalised.
Ignore symlinks when printing help
Use regexes to parse help and usage
Use regexes to parse summaries
Account for underscores in prepare script
Remove sub-sh-shell, no longer needed
Add space before arrow for help on containers
Bring README up to date