Support more complex env invocations in shebang lines

Support more complex env invocations in shebang lines

This is not an exact re-implementation of what env accepts, but
hopefully it should be "good enough".

Example of known limitation: we assume that arguments for
--long-options will be set with '=', but that is not
necessarily the case.  '--unset' (mandatory argument) can be
passed as '--unset=VAR' or '--unset VAR', but
'--default-signal' (optional argument) requires an '=' sign.

For bug#64939.

* lisp/files.el (auto-mode-interpreter-regexp): Account for
supplementary arguments passed beside -S/--split-string.
* test/lisp/files-tests.el (files-tests-auto-mode-interpreter):
Test some of these combinations.