afl-fuzz debugging support.

afl-fuzz debugging support.

Includes:

- SIGABRT in case of Host_Error calls (these would normally "safely"
  exit the dedicated server, so remote exploits calling this are
  interesting).
- SIGXCPU in case NetConn_ServerFrame is hit again after the injected
  packet (shouldn't ever happen - just in case, as it also would
  indicate a remote exploitable problem as remaining sockets in the list
  were skipped then).
- Facility "net_dump" to dump all incoming network packets to files.
  Allows to easily choose an "interesting" one.
- Facility "net_inject" to replace a specific packet by the content of a
  file. Includes a flag net_inject_index, which causes the engine to
  exit successfully after the injected packet was processed.
  Note that successful injection requires the following engine flags:
  -nosound +sys_usenoclockbutbenchmark 1 +cl_maxfps 30 +sv_random_seed 0
  to remove any nondeterministic influence (including external timing
  sources).
- After injecting, the loop is executed that crashes in Xonotic bug
  report https://gitlab.com/xonotic/darkplaces/issues/24 - this helps to figure
  out said bug report.
- afl-fuzz instrumentation (and thus afl-fuzz's fork server) is started
  only just before injection. This keeps afl-fuzz runs fast (about
  70/sec), skipping the otherwise unavoidable loading time of 20
  seconds.
- Disable vid_glx's signal handler, so afl-fuzz will actually see
  crashes as crashes (and as fast as possible - in my testing crashes
  were sometimes misdetcted as hangs as the signal handler, which tries
  a clean exit, will acquire mutexes held during the crash).