Add execvpe support

[Susurrus]

This was removed in #681 because it was broken. Re-adding it is basically undoing fcc22f2 (without the feature support and instead conditionally including it on supported platforms). libc has execvpe included for haiku, linux, and openbsd platforms. If you are a new contributor, go ahead and read CONTRIBUTING.md, assign this to yourself, and file a PR when you've gotten somewhere (start the title with WIP if it's not ready to merge, and include "closes #682" in the text of the PR).

[panzertime]

I'd like to be assigned this issue

[Susurrus]

Awesome! Thanks for offering to contribute to nix!

I wasn't expecting someone to pick this up so quickly, so this is actually missing a crucial detail that you'll want to make a first commit that changes the libc dependency back to git (making that one change as a single commit means it can be easily removed later). And since you got here so quick I also wanted to ask how did you find out about this issue or nix?

Just post here if you have any issues and I'll be sure to address them ASAP!

[panzertime]

Basically, I stumbled across nix while searching for a way to practice my Rust, and when I looked at the issues I was lucky enough to find a fresh beginner issue

[Susurrus]

Awesome, well I'm glad I keep a few of these around! There's plenty of easy work to do around here (see #254 and #264 for more).

[F1rst-Unicorn]

Hi guys,
I have ported the code from the old pull request and adapted the missing parts. Will be glad to see this
merged soon.