ivrd: fflush() stdout before entering esl_listen()

Fixes repeatedly printing the 'Starting forking listener' message,
if stdout has been redirected into a logfile (for example).

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>
esl: Don't abort the esl_listen() loop(s) if accept() returns with errno == EINTR

ivrd doesn't use sigaction + SA_RESTART for SIGCHILD. An exiting
child process will interrupt accept() in the parent to handle
the signal, which makes accept() return -1 (errno = EINTR) after
the sighandler function returns.

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>