use leaky bucket algorithm to limit connection rate

retry hosts which did not return any data

more scanners besides TCP:
  * SMB
  * Sun RPC
  * DNS (version and open resolver)
  * spawn arbitrary programs after connection has been established
  * SMTP relay tester

fork() subprocesss to keep the number of file descriptors per process
at a reasonable level (maybe not needed with /dev/epoll)

SSL support

# arch-tag: 2353dcb1-6ae2-4801-a474-9523ea85c7f1