Allow Win32 to compile under MinGW. Major changes are:

        Win32 port is now called 'win32' rather than 'win'
        add -lwsock32 on Win32
        make gethostname() be only used when kerberos4 is enabled
        use /port/getopt.c
        new /port/opendir.c routines
        disable GUC unix_socket_group on Win32
        convert some keywords.c symbols to KEYWORD_P to prevent conflict
        create new FCNTL_NONBLOCK macro to turn off socket blocking
        create new /include/port.h file that has /port prototypes, move
          out of c.h
        new /include/port/win32_include dir to hold missing include files
        work around ERROR being defined in Win32 includes