env: add --env0-from option

[krosci]

Add the --env0-from=FILE option from GNU coreutils 9.12. It reads NUL-terminated name=value entries from FILE (or standard input when FILE is -) and applies them to the environment.

With --ignore-environment the file entries become the environment itself, preserving order, duplicates and entries without a = byte, so env -i --env0-from=f --null round-trips exactly and the entries reach the exec'd command untouched. Without -i the entries are merged into the inherited environment, with name=value entries overriding existing variables and entries without = appended verbatim. Multiple --env0-from files are processed in the order given.

Closes #14602

[krosci]

@cakebaker could you take a look at it? Thanks!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Skip an intermittent issue tests/cut/bounded-memory (was skipped on 'main', now failing)
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[sylvestre]

The ci is red!