Simplify checks for xdg-open and xdg-email

Simplify checks for xdg-open and xdg-email

browse-url's xdg-open detection was too picky on some GNU/Linux
desktops; see Bug#25778.  Simplify the code by assuming xdg-open works
if it is executable, as nowadays this is more likely to be correct than
trying to use heuristics from a few years ago.  Don't test for nohup: it
is ineffective nowadays, as xdg-open's child uses the default action for
SIGHUP even if xdg-open's invoker ignores SIGHUP.  While we're at it,
allow for Wayland here, as "emacs -nw" might be running in a non-X
Wayland terminal.
* lisp/mail/emacsbug.el (report-emacs-bug-can-use-xdg-email):
* lisp/net/browse-url.el (browse-url-can-use-xdg-open):
Simplify to a test for DISPLAY and whether the helper program is
executable.  Allow WAYLAND_DISPLAY as an option.