common,msg: define SOCKOPT_VAL_TYPE

common,msg: define SOCKOPT_VAL_TYPE

On Windows, the "optval" argument of the setsockopt function is
a char pointer, as opposed to a void pointer on other platforms.

In order to avoid compilation issues, we'll do an explicit cast
using a conditionally defined type.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>