Make behavior of environment.opIndexAssign on Posix to match the behavior on Windows

[FreeSlave]

Setting null as environment value leads to removing this variable
according to SetEnvironmentVariable documentation while Posix leaves
unspecified what setenev should do if value is null.

[wilzbach]

This is a simple change and I don't have a strong opinion about this, but as no one has bother to look at this so far, here are a couple of comments:

according to SetEnvironmentVariable documentation while Posix leaves unspecified what setenev should do

For the lazy ones: https://msdn.microsoft.com/en-us/library/windows/desktop/ms686206(v=vs.85).aspx

=> The idea of unifying the behavior seems appealing, it comes with a very low cost and probably doesn't hurt anyone.

Has anyone a (strong) opinion about this?

[wilzbach]

nit: null as its code as well

[JackStouffer]

This is a bug fix, as current behavior on posix is sending a SIGTERM when null is set.

[wilzbach]

Vibe.d seems to have a transient error?

Received string message: Bye bye
Received iii: 1 2 3
Receive loop finished.
Receiver watchdog failure.
uncaught exception
dwarfeh(224) fatal error
Program exited with code -6
script returned exit code 2

Does the new Project Tester already support restarting builds or do we need to kick him manually with a git rebase?

[JackStouffer]

I don't see a way to restart it.

@MartinNowak Can return Jenkins to not required for merging until it's more stable?

[wilzbach]

I don't see a way to restart it.

I rebased the PR manually, s.t. hopefully it can be auto-merged.