Use execvpe to execute zenity and kdialog

[paulfd]

This removes the hard-coding of zenity or kdialog paths. Zenity is preferred still.

[scheffle]

@paulfd: Is 'execvpe' available on BSD? We have to make sure it works there too.

[paulfd]

I'll check it out, are there any particular version/OS you're targeting?

[paulfd]

Apparently it's a problem on FreeBSD so you were right to be wary! If you're OK in principle for the patch I can look into it to find a suitable, portable solution.

[scheffle]

Sure, a portable solution would be nice! Thanks.

[redtide]

I rebased the code to a recent commit, left the original execve for BSD and execvpe for Linux, which helps NixOS like distributions to find the KDialog/Zenity paths.