As described in Google's style guide, the basis for Neovim's
> All of a project's header files should be listed as descendants of the
> project's source directory without use of UNIX directory shortcuts .
> (the current directory) or .. (the parent directory).
Add src as an include directory to facilitate this.
* 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
