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