Experiments with using C-level co-routines (like Posix swapcontext()) to get
non-blocking operation from libmysqlclient.

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Here is a small example of suggested non-blocking API:


    static void
    wait_for_mysql(MYSQL *mysql, int status)
    {
      struct pollfd pfd;
      p.fd= mysql_get_socket_fd(&mysql);
      p.events=
	(status & MYSQL_WAIT_READ ? POLLIN : 0) |
	(status & MYSQL_WAIT_WRITE ? POLLOUT : 0);
      poll(&pfd, 1, 0);
    }


    status= mysql_real_connect_start(&ret, &mysql, "localhost", "test", "testpass", "test",
				     0, NULL, 0);
    while (status)
    {
      wait_for_mysql(&mysql, status);
      status= mysql_real_connect_cont(&ret, &mysql);
    }


    status= mysql_real_query_start(&err, &mysql, SL("SHOW STATUS"));
    while (status)
    {
      wait_for_mysql(&mysql, status);
      status= mysql_real_query_cont(&err, &mysql);
    }


    status= mysql_fetch_row_start(&row, res);
    while (status)
    {
      wait_for_mysql(&mysql, status);
      status= mysql_fetch_row_cont(&row, res);
    }


Idea is that if a call R=foo(...) can block, we introduce two more calls

    S=foo_start(&R, ...)
    S=foo_cont(&R, ...)


S (status) returns 0 is the call is done; then R is set to the return value
from the non-blocking call. When S returns non-zero, then the call is blocking
on some condition; individual bits in S say what we are waiting for,
eg. MYSQL_WAIT_READ or MYSQL_WAIT_WRITE.