socket-util: Introduce send_one_fd_iov() and receive_one_fd_iov()
    
    These take a struct iovec to send data together with the passed FD.
    
    The receive function returns the FD through an output argument. In case data is
    received, but no FD is passed, the receive function will set the output
    argument to -1 explicitly.
    
    Update code in dynamic-user to use the new helpers.
    
    (cherry picked from commit d34673ecb825aa9ecf6958b0caab792f5061c56a)
    
    Resolves: #1683319
    
    patch_name: 0190-socket-util-Introduce-send_one_fd_iov-and-receive_on.patch
    present_in_specfile: true
    location_in_specfile: 190
    squash_commits: true