Hello list,

I would like to suggest an enhancement for the proc:system/execute/fork
family of functions: it should be possible to control the environment
variables of the external process. At the moment, the environment is
always inherited from the calling basex process, as far as I can tell.

There are several reasons why it is desirable to control the environment
of external processes:

* set variables to values that are not known statically when starting basex
* avoid information leaking (restrict environment to the minimal
required subset)
* control the PATH of the external process

It would be great to have an additional, optional entry in the $options
map (maybe named "env" or "environment") that contains a map of
environment variables for the external process. If absent, the current
behavior (inherited environment) should be used. If the option is given,
**only** the environment variables in the map should be used for the
external process.

Effectively, the "env"/"environment" option would behave as if it had a
default value of `map:merge( available-environment-variables() !
map:entry(., environment-variable(.)) )`

The ProcessBuilder in Java should allow for a straightforward
implementation.

What do you think?
Cheers,
Hauke