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/main.c

@@ -818,7 +818,7 @@ static void init_locale(void)
 # endif
 
   {
-    int mustfree = FALSE;
+    bool mustfree = false;
     char_u  *p;
 
     /* expand_env() doesn't work yet, because chartab[] is not initialized