bug#25778: 25.1; [PATCH] Drastically simplify xdg-open check

[Glenn Morris]

The area seems a mess, but:

1) nohup was removed from browse-url-xdg-open in 1c0f1a199ca, with no
explanation and as part of a change that seems unrelated.

2) In https://debbugs.gnu.org/9779#20, the author of that change said

    I note that since 22-Jun-2011 browse-url-xdg-open no longer uses
    nohup.  This is either a mistake on my part (I didn't read the
    comment in browse-url-can-use-xdg-open) or it is indeed
    unnecessary and therefore isn't a requirement for xdg-open.

3) In reply, Jan said in https://debbugs.gnu.org/9779#29

    See discussion at
    http://lists.gnu.org/archive/html/emacs-devel/2009-07/msg00279.html.
    nohup is still needed for xdg-open.

And AFAICS that was the last comment about nohup. No-one ever seems to
have followed up Jan's comment.