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/ex_cmds.c

@@ -5059,7 +5059,7 @@ void fix_help_buffer(void)
   char_u      *fname;
   char_u      *p;
   char_u      *rt;
-  int mustfree;
+  bool mustfree;
 
   /* set filetype to "help". */
   set_option_value((char_u *)"ft", 0L, (char_u *)"help", OPT_LOCAL);