dccp sender: Keep polling until an entry becomes free (Gerrit Renker).

If write() on the dccp fd fails with EAGAIN, the current code retries
100 times and then gives up, returning -E_DCCP_WRITE which causes a
shutdown of the dccp connection.

CCID3 has a sending wait queue which fills up and is emptied
asynchronously. The EAGAIN case means that there is currently no space
in the wait queue, but this can change at any moment and is thus not
an error condition. So keep polling until an entry becomes free.