os/env: use libuv v1.12 getenv/setenv API

- Minimum required libuv is now v1.12
- Because `uv_os_getenv` requires allocating, we must manage a map
  (`envmap` in `env.c`) to maintain the old behavior of `os_getenv` .
- free() map-items after removal. khash.h does not make copies of
  anything, so even its keys must be memory-managed by the caller.

closes #8398
closes #9267

src/nvim/path.c

@@ -2271,7 +2271,7 @@ int path_is_absolute(const char_u *fname)
 void path_guess_exepath(const char *argv0, char *buf, size_t bufsize)
   FUNC_ATTR_NONNULL_ALL
 {
-  char *path = getenv("PATH");
+  const char *path = os_getenv("PATH");
 
   if (path == NULL || path_is_absolute((char_u *)argv0)) {
     xstrlcpy(buf, argv0, bufsize);