Use irc_write_all() to send the message in bitlbee_shutdown

This ensures that the message arrives by using the "now" parameter of
irc_write_all(), because in my testing it didn't show up (even though
surrounding it with printf() worked, in inetd mode)
Improved signal handling to avoid deadlocks

- SIGSEGV: broadcast a message manually, avoiding the usual irc_write()
  functions which are unsafe due to malloc().
- SIGTERM, SIGINT: Write to a pipe which gets handled in the main loop by
  bitlbee_shutdown(), saving configs and stuff.
- SIGCHLD: set to ignore explicitly, which handles zombies correctly.
  This also drops some log messages with 'info' level, which in practice
  means they never got logged.
- SIGPIPE: set to ignore (nobody cares)
- SIGILL, SIGBUS, SIGFPE, SIGQUIT, SIGXCPU: Not handling anymore.