env: Add a handful of test cases.

MFC after: 3 days
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D46996

(cherry picked from commit 334af5e4131b21c

env: Add a handful of test cases.

MFC after: 3 days
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D46996

(cherry picked from commit 334af5e4131b21c658203635bf713d6a59846585)

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

(cherry picked from commit a0dfb0668b45506de97beb4c7acbe3fd1ba69fc8)

env: Add an option to change the directory.

This mirrors the equivalent option in GNU coreutils env, but does not
add support for long options.

MFC after: 3 days
Relnotes: yes
Sponsored by: Klara, Inc.
Reviewed by: 0mp, bcr
Differential Revision: https://reviews.freebsd.org/D47008

(cherry picked from commit 6f6166e49c78f6460732c02bbbba6fcc218221cf)

env: Check the status of stdout.

MFC after: 3 days
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D47009

(cherry picked from commit c2d93a803acef634bd0eede6673aeea59e90c277)

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