fix(pam): use environ variable when getenv doesn't work

[saidsay-so]

On some programs like sudo, getenv doesn't return the environment variable value. We check the char **environ variable also to not miss those values.

Close #902

[principis]

This doesn't really make sense to me. Why is it that environ may contain variables that getenv doesn't return? It should be the same.

[saidsay-so]

This doesn't really make sense to me. Why is it that environ may contain variables that getenv doesn't return? It should be the same.

Frankly I don't know myself, I wasn't thinking that it was possible but I was able to reproduce the bug as it was initially reported and the fix works.