Fraser Tweedale authored Jan 10, 2021 and Marge Bot committed Jul 06, 2021

System.Environment.getExecutablePath has some problems:

- Some system-specific implementations throw an exception in some
  scenarios, e.g. when the executable file has been deleted

- The Linux implementation succeeds but returns an invalid FilePath
  when the file has been deleted.

- The fallback implementation returns argv[0] which is not
  necessarily an absolute path, and is subject to manipulation.

- The documentation does not explain any of this.

Breaking the getExecutablePath API or changing its behaviour is not
an appealing direction.  So we will provide a new API.

There are two facets to the problem of querying the executable path:

1. Does the platform provide a reliable way to do it?  This is
   statically known.

2. If so, is there a valid answer, and what is it?  This may vary,
   even over the runtime of a single process.

Accordingly, the type of the new mechanism is:

  Maybe (IO (Maybe FilePath))

This commit implements this mechanism, defining the query action for
FreeBSD, Linux, macOS and Windows.

Fixes: #10957
Fixes: #12377