refactor: split some more functions from misc1.c

This commit pulls the some environment-variable handling functions out of
misc1.c and in to os/env.c. Previously submited as #1231, this is the start of
a patch series that does that work based on a more up-to-date master branch.

Major tasks accomplished:
  - move functions and fix includes
  - fix clint/clang analysis warnings
  - correct documentation comments

src/nvim/log.c

@@ -8,7 +8,6 @@
 #include <unistd.h>
 
 #include "nvim/log.h"
-#include "nvim/misc1.h"
 #include "nvim/types.h"
 #include "nvim/os/os.h"
 #include "nvim/os/time.h"