env: add -a,--argv0 to set the first argument passed to exec

env: add -a,--argv0 to set the first argument passed to exec

Using the shell's exec -a feature can be awkward
so add support for setting overriding argv[0].
This gives env full control over the arguments it passes.

* src/env.c: Accept -a,--argv0 and set argv[0] appropriately.
* tests/env/env.sh: Add test cases.
* doc/coreutils.texi (env invocation): Describe -a,--argv0.
* NEWS: Mention the new feature.