With this change, we can formalize the renaming of a parameter while
maintaining the old name temporarily for compatibility.

A deprecated alias can be set with either param.set or the -p option,
but won't be listed by:

- param.show [-j]
- param.show [-j] changed
- param.show -l

Only an explicit param.show for the name of the alias will provide a
minimal documentation with a deprecation notice and the current value.
In the manual, there is only a deprecation notice. The rationale is
that administration tools shouldn't pick them up when enumerating the
parameters.

Since we currently don't have deprecated parameters, this can only be
tested manually, for example:

    PARAM_ALIAS(vcl_dir, vcl_path)

To ensure that we don't break this, we could consider having a perpetual
deprecated parameter.