gpg: Improve the photo image viewer selection.

* g10/exec.c (w32_system): Add "!ShellExecute" special.
* g10/photoid.c (get_default_photo_command): Use the new ShellExecute
under Windows and fallbac to 'display' and 'xdg-open' in the Unix
case.
(show_photos): Flush stdout so that the output is shown before the
image pops up.

For Unix this basically syncs the code with what we have in gpg 1.4.
Note that xdg-open may not be used when running as root which we
support here.

For Windows we now use ShellExecute as this seems to be preferred over
"cmd /c start"; however this does not solve the actual problem we had
in the bug report. To solve that problem we resort to a wait
parameter which defaults to 400ms. This works on my Windows-10
virtualized test box. If we can figure out which simple viewers are
commonly installed on Windows we should enhance this patch to test for
them.

• GnuPG-bug-id: T4334
• Signed-off-by: Werner Koch <wk@gnupg.org>