Sets options on a socket.
    The $QIO equivalent is the IO$_SETMODE function.
    Format
      #include  <types.h>
      #include  <socket.h>
      int setsockopt  ( int s, int level, int optname, char *optval,
                      int optlen ); (_DECC_V4_SOURCE)
      int setsockopt  ( int s, int level, int optname, const void
                      *optval, size_t optlen ); (not_DECC_V4_SOURCE)

1  –  Arguments

 s
    A socket descriptor created by the socket() function.
 level
    The protocol level for which the socket options are to be
    modified. It can have one of the following values:
    SOL_SOCKET         Set the options at the socket level.
    p                  Any protocol number. Set the options for
                       protocol level p. For IPv4, see the IN.H
                       header file for the IPPROTO values. For IPv6,
                       see the IN6.H header file for the IPPROTO_IPV6
                       values.
 optname
    Interpreted by the protocol specified in level. Options at each
    protocol level are documented with the protocol.
    Refer to:
    o  Socket Options for a list of socket options
    o  TCP Protocol Options for a list of TCP options
    o  IP Protocol Options for a list of IP options
 optval
    Points to a buffer containing the arguments of the specified
    option.
    All socket-level options other than SO_LINGER should be nonzero
    if the option is to be enabled, or zero if it is to be disabled.
    SO_LINGER uses a linger structure argument defined in the
    SOCKET.H header file. This structure specifies the desired state
    of the option and the linger interval. The option value for the
    SO_LINGER command is the address of a linger structure.
    If the socket is type SOCK_STREAM, which promises the reliable
    delivery of data, and l_onoff is nonzero, the system blocks the
    process on the close() attempt until it is able to transmit the
    data or until it decides it is unable to deliver the information.
    A timeout period, called the linger interval, is specified in l_
    linger.
    If l_onoff is set to zero and a close() is issued, the system
    processes the close in a manner that allows the process to
    continue as soon as possible.
 optlen
    An integer specifying the size of the buffer pointed to by
    optval.

2  –  Description

    This function manipulates options associated with a socket.
    Options can exist at multiple protocol levels. They are always
    present at the uppermost socket level.
    When manipulating socket options, specify the level at which the
    option resides and the name of the option. To manipulate options
    at the socket level, specify the value of level as SOL_SOCKET. To
    manipulate options at any other level, supply the protocol number
    of the appropriate protocol controlling the option. For example,
    to indicate that an option is to be interpreted by TCP, set the
    value for the level argument to the protocol number (IPPROTO_TCP)
    of TCP.
    For IPv4, see the IN.H header file for the various IPPROTO
    values. For IPv6, see the IN6.H header file for the various
    IPPROTO_IPV6 values.

3  –  Return Values

    0                  Successful completion.
    -1                 Error; errno is set to indicate the error.

4  –  Errors

    EACCES             The calling process does not have appropriate
                       permissions.
    EBADF              The descriptor is invalid.
    EDOM               The send and receive timeout values are too
                       large to fit in the timeout fields of the
                       socket structure.
    EINVAL             The optlen argument is invalid.
    EISCONN            The socket is already connected; the specified
                       option cannot be set when the socket is in the
                       connected state.
    EFAULT             The optval argument is not in a readable part
                       of the user address space.
    ENOBUFS            The system had insufficient resources to
                       complete the call.
    ENOPROTOOPT        The option is unknown.
    ENOTSOCK           The socket descriptor is invalid.
    EFAULT             The optname argument is invalid.