Use <pulsecore/socket.h> instead of <sys/socket.h>

Use <pulsecore/socket.h> instead of <sys/socket.h>

The check whether POSIX socket.h or WIN32 winsock2.h must be included can be
made centrally.  The downside is that some functionality of e.g. arpa/inet.h is
also implemented in winsock.h, so that some files that don't use socket
functions, but do use inet.h functions, must also include pulsecore/socket.h.
(as well as arpa/inet.h)