Implement improved "get executable path" query

TODO:

• test the change on
  • linux
  • freebsd
  • macos
  • windows
• add tests to test suite
• update getExecutablePath documentation
• decide and document getExecutablePath deprecation strategy/schedule
• bikeshed the name of the new interface

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 (closed) Fixes: #12377 (closed)