Post get_envs library methods
Copy Meterpreter return hash

Dont add a key if no value is found
Land #2961, get_env and get_envs Post mixin

This unbreaks the changes introduced by #2782 by introducing
get_env and get_envs for shell sessions (not just meterpreter sessions).
Fix linux download_exec for #2961

Note! This module already seems pretty broken, in that it doesn't appear
to correctly locate curl or wget. Will open another bug on that.

[See RM #8777]
Land #3129 - Fix 2782 with 2961 and stop stack-tracing download_exec