env: Improve documentation.

* The `env` utility's inability to run a command whose name contains an
  equal sign is a feature, not a bug, so move that paragraph up from the
  BUGS section to the DESCRIPTION section.

* Mention that this can be worked around by prefixing the command name
  with `command`, and add an example of this to the EXAMPLE section.

* Add a test case which verifies that `env` does not run a command with
  an equal sign in its name even if it exists, and also demonstrates the
  workaround.

MFC after:	3 days
Sponsored by:	Klara, Inc.
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D46997

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