fix for open_port env vars with trailing '=' characters

[Steve Vinoski]

Passing {env, Env} to open_port with an environment variable value
containing a trailing '=' character results in the variable not being
set into the environment of the new process. Affects only UNIX as far
as I can see. Here's a fix:

git fetch git://github.com/vinoski/otp.git env_with_equal_sign

For more details, here's the full commit message:

http://github.com/vinoski/otp/commit/0a6db8663b5558c152a351075bff296733bd3a0d

--steve