kern_environment: Give the static environment a chance to disable MD env

This variable has been given the name "loader_env.disabled" as it's the
primary way most people will have an MD environment. This restores the
previously-default behavior of ignoring the loader(8) environment, which may
be useful for vendor distributions or other scenarios where inheriting the
loader environment may be considered a security issue or potentially
breaking of a more locked-down environment.

As the change to config(5) indicates, disabling the loader environment
should not be a choice made lightly since it may provide ACPI hints and
other useful things that the system can rely on to boot.

An UPDATING entry has been added to mention an upgrade path for those that
may have relied on the previous behavior.

Discussed with: bde
Relnotes: yes (maybe)