Add "environ"

[safinaskar]

Please, reexport UNIX's variable environ

https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html

[joshtriplett]

There are currently many issues being discussed about environment handling and concurrency. If we add this, we need to make sure it's unsafe to access.

[evmar]

It's a *const *mut libc::c_char so any use of the pointer its already unsafe (?)