env: add --chdir option

env: add --chdir option

This is useful when chaining with other commands that run commands in a
different context, while avoiding using the shell to cd, and thus
having to consider shell quoting the chained command.

* NEWS (New features): Document the new option.
* doc/coreutils.texi (env invocation): Likewise.
* src/env.c (usage): Likewise.
(main): Implement the new option.
* tests/misc/env.sh: Test the new option.