ARM-8 Replace comma with 'or' and allow 'and' as simple nesting.

It was hard to understand that when with comma separated list had
the meaning of "or". To make the language symmetrical, an "and"
is now also allowed with the same effect as a simple nesting.
ARM-8 Clarify that environments.pp file is evaluated on each request.
ARM-8 Add bindingsdir to environment defaults

Bindingsdir is the directory under which bindings are stored.
ARM-8 Add note about parameter unit mapping

Parameter mapping of the type {a => a, b => b} is a corner case
that could be supported with better concrete logic, but it is
expensive as it "destroys" bind map expressions. 

This commit adds a todo note about this consideration.