env: add an opaque pointer for implementation use

This is a request from an implementer of an alternate (non-DPDK) env
implementation; it will only have meaning for the specific env
implementaiton, but it can be used for passing env-specific arguments
through the generic spdk_env_init() call.

Change-Id: I37f70dd3b961f08c3bc2f25f3d12cc02bd3d6699
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/382055


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>