- 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#8398closes#9267