MINOR: ext-check: add an option to preserve environment variables

MINOR: ext-check: add an option to preserve environment variables

In Github issue #2128, @jvincze84 explained the complexity of using
external checks in some advanced setups due to the systematic purge of
environment variables, and expressed the desire to preserve the
existing environment. During the discussion an agreement was found
around having an option to "external-check" to do that and that
solution was tested and confirmed to work by user @nyxi.

This patch just cleans this up, implements the option as
"preserve-env" and documents it. The default behavior does not change,
the environment is still purged, unless "preserve-env" is passed. The
choice of not using "import-env" instead was made so that we could
later use it to name specific variables that have to be imported
instead of keeping the whole environment.

The patch is simple enough that it could be backported if needed (and
was in fact tested on 2.6 first).