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/hashtab.h

@@ -19,7 +19,7 @@ typedef size_t hash_T;
 #define HASHITEM_EMPTY(hi) ((hi)->hi_key == NULL \
                             || (hi)->hi_key == (char_u *)&hash_removed)
 
-/// A hastable item.
+/// Hashtable item.
 ///
 /// Each item has a NUL terminated string key.
 /// A key can appear only once in the table.