bug#17425: 24.4.50; tramp-cleanup-this-connection on push-button

[Stefan Monnier]

> (let ((default-directory "http://debbugs.gnu.org";))
>   (url-handler-mode 1)
>   (browse-url "17425"))

For the above to work (assuming browse-url spawns an external process),
browse-url will have to pass the default-directory to the process.
It can't do it using POSIX's "current working directory", so it will
have to pass it some other way.  Once that's done, browse-url can use
unhandled-file-name-directory and change default-directory without
breaking anything.

I.e. unhandled-file-name-directory should be used close to the
call-process/process-start, and for it to work we need to make sure none
of the parameters are relative file-names, indeed.  But that's the only
way it can work reliably, since there's no way to pass relative file
names to a process when the base directory doesn't exist in the OS.


        Stefan