Where regular expression is read from a variable and -e argument is not
provided, the resulting call may end up being interpreted as grep
argument.

See also:

    https://github.com/koalaman/shellcheck/issues/1342

Action won't start if one is already running.  This should prevent
overlapping actions.

It's possible to turn off locking to allow running imapdomo as service,
ie. locking will be taken care of by init system.

For code readability and maintenance

Having just 4 valid actions (albeit they do form a meaningful workflow)
was rather historical limitation caused by author's negligence.
User should be able to define any actions they want by simply creating
handlers for them.

Later we might want to provide a guidance (template) for that, but
that's a TODO item.

The host split turned to be a vast overengineering.