Do not unconditionally fork() after accept(). accept() can
return -1 due to an interrupted system call (i.e. SIGCHLD).
If we fork in that case ftpd can get into an
accept()/SIGCHLD/fork/[fail]/repeat loop.
Reported-by: fabian <fabian.duelli@bluewin.ch>
Obtained from: DragonflyBSD
MFC after: 1 month