Cygwin: Allow executing Windows Store's "app execution aliases"

The Windows Store version of Python (and apparently other Windows Store
applications) install a special reparse point called "app execution
alias" into the user's `PATH`.

These applications can be executed without any problem, but they cannot
be read as if they were files. This trips up Cygwin's beautiful logic that
tries to determine whether we're about to execute a Cygwin executable or
not: instead of executing the application, it will fail, saying
"Permission denied".

Let's detect this situation (`NtOpenFile()` helpfully says that this
operation is not supported on this reparse point type), and simply skip
the logic: Windows Store apps are not Cygwin executables (and even if
they were, it is unlikely that they would come with a compatible
`cygwin1.dll` or `msys-2.0.dll`).

This fixes https://github.com/msys2/MSYS2-packages/issues/1943

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/spawn.cc

@@ -1276,6 +1276,13 @@ av::setup (const char *prog_arg, path_conv& real_path, const char *ext,
 			     FILE_SYNCHRONOUS_IO_NONALERT
 			     | FILE_OPEN_FOR_BACKUP_INTENT
 			     | FILE_NON_DIRECTORY_FILE);
+	if (status == STATUS_IO_REPARSE_TAG_NOT_HANDLED)
+	  {
+	    /* This is most likely an app execution alias (such as the
+	       Windows Store version of Python, i.e. not a Cygwin program */
+	    real_path.set_cygexec (false);
+	    break;
+	  }
 	if (!NT_SUCCESS (status))
 	  {
 	    /* File is not readable?  Doesn't mean it's not executable.