cleanup compat.h Windows stuff

- remove an unneded include for mswsock.h as we use winsock2.h anyway
- move typedef u_int SOCKET; into the #ifndef WIN32 part
- remove typedef int socklen_t;  as this is defined in ws2tcpip.h
Merge pull request #2857 from Diapolo/compat_cleanup

cleanup compat.h Windows stuff