forbid access to environment variables

[Sorin Marian Nasoi]

As suggested in F&O spec in the fn:environment-variable section Notes:

"Security advice: Queries from untrusted sources should not be permitted unrestricted access to environment variables. For example, the name of the account under which the query is running may be useful information to a would-be intruder. An implementation may therefore choose to restrict access to the environment, or may provide a facility to make fn:environment-variable always return the empty sequence."

The user should have a way of forbidding access to environment variables, meaning making fn:available-environment-variables function return empty sequence.

The test-cases impacted by this issue are:
- 'fn-available-environment-variables-011' from 'fn-available-environment-variables' test-set
- 'environment-variable-005', 'environment-variable-006', 'environment-variable-007' from 'fn-environment-variable' test-set