Fix style issues.

* Rename mch_full_name to mch_get_absolute_path.
* Rename mch_is_full_name to mch_is_absolute_path.
* Add a lot of missing parentheses.
* Remove yoda-conditions for consistency.
* Remove spaces in function declaration.
cleanup environment variable handling + unit tests

  * 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