urxvt leaks DESKTOP_STARTUP_ID to children

[Dimitrios Semitsoglou-Tsiapos]

Hello,

I have recently had trouble xmbedding the `zathura` PDF viewer into
`tabbed` when running it via `urxvt` that was spawned using `awesomewm`.
`zathura` was failing to show any content. You can find the bug report I
opened for it on their bug tracker[1].

On the fourth comment, Sebastian Ramacher explains why this appears to
be a bug with `urxvt`, which should be unsetting this variable:

> Then I don't think that's something we can fix on zathura's side. From my understanding of the startup notification spec, it's a bug between awesome and urxvt that DESKTOP_STARTUP_ID is still visible to zathura although it wasn't intended for zathura. It seems like a bug to me, that urxvt leaks its DESKTOP_STARTUP_ID to zathura. The spec reads:
> 
>     It is suggested to unset this environment variable in the launchee as soon as it's read, to avoid possible reuse by some process started later by launchee.
> 
> I can't find any code in urxvt that does that, even though it appears to offer startup notification support.
I am not certain whether this is the proper fix for the bug I was
dealing with, but it does resolve it.

Best regards,
Dimitrios Semitsoglou-Tsiapos

[1] https://bugs.pwmt.org/project/zathura/issue/750