Signal handling

[gwolf-011235]

signal() is in the allowed functions so it seems to be expected that we handle signals.

The main signal would be SIGINT for server shutdown. This could be used to shutdown the server gracefully.

Possible implementation

• Since signal handlers shouldn't do that much, a global variable of type volatile std::sig_atomic_int could be flipped.
• The multiplexer needs to check this variable to check if he should shutdown.
  • caveat: epoll_wait() returns -1 in case of a signal with errno EINTR. So this needs also be checked.
  • Problem: if timeout is set to -1 (is this something we want to expose to configuration?) we never check shutdown.
• In the case of shutdown every connected client should receive a notification.
• Problem: what to do with other signals?