* removed a putenv() implementation which isn't needed anymore
* mch_getenv() and mch_setenv() are now functions in src/os/env.c
* removes direct calls to getenv() and setenv() outside of src/os/env.c
* refactored the logic of get_env_name into mch_getenvname_at_index
* added unittests for the functions in os/env.c