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/map.h

@@ -25,11 +25,15 @@
   void map_##T##_##U##_free(Map(T, U) *map); \
   U map_##T##_##U##_get(Map(T, U) *map, T key); \
   bool map_##T##_##U##_has(Map(T, U) *map, T key); \
+  T map_##T##_##U##_key(Map(T, U) *map, T key); \
   U map_##T##_##U##_put(Map(T, U) *map, T key, U value); \
   U *map_##T##_##U##_ref(Map(T, U) *map, T key, bool put); \
   U map_##T##_##U##_del(Map(T, U) *map, T key); \
   void map_##T##_##U##_clear(Map(T, U) *map);
 
+//
+// NOTE: Keys AND values must be allocated! khash.h does not make a copy.
+//
 MAP_DECLS(int, int)
 MAP_DECLS(cstr_t, ptr_t)
 MAP_DECLS(ptr_t, ptr_t)
@@ -43,6 +47,7 @@ MAP_DECLS(String, handle_T)
 #define map_free(T, U) map_##T##_##U##_free
 #define map_get(T, U) map_##T##_##U##_get
 #define map_has(T, U) map_##T##_##U##_has
+#define map_key(T, U) map_##T##_##U##_key
 #define map_put(T, U) map_##T##_##U##_put
 #define map_ref(T, U) map_##T##_##U##_ref
 #define map_del(T, U) map_##T##_##U##_del
@@ -52,7 +57,9 @@ MAP_DECLS(String, handle_T)
 #define pmap_free(T) map_free(T, ptr_t)
 #define pmap_get(T) map_get(T, ptr_t)
 #define pmap_has(T) map_has(T, ptr_t)
+#define pmap_key(T) map_key(T, ptr_t)
 #define pmap_put(T) map_put(T, ptr_t)
+/// @see pmap_del2
 #define pmap_del(T) map_del(T, ptr_t)
 #define pmap_clear(T) map_clear(T, ptr_t)
 
@@ -62,4 +69,6 @@ MAP_DECLS(String, handle_T)
 #define map_foreach_value(map, value, block) \
   kh_foreach_value(map->table, value, block)
 
+void pmap_del2(PMap(cstr_t) *map, const char *key);
+
 #endif  // NVIM_MAP_H