Add IPv6 support

Added argument for the bind address to use.

Add syslog support.

- Correct some exit codes.
- #include <syslog.h> can't be removed since we use LOG_* defines.
- syslog/openlog/closelog do not need any special libraries.

Correct possible hangs.

In certain condition, if the second call to splice does not write a full
packet, the first call to splice can block since the pipe is full, to prevent
this condition, we never block while writing to the pipe.

Updating the connection count via systemd in child handler can cause a
deadlock in certain conditions.

(gdb) bt
*0  __lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
*1  0x00007f6aa9a5752f in _L_lock_10524 () from /lib/x86_64-linux-gnu/libc.so.6
*2  0x00007f6aa9a5515f in malloc_atfork (sz=140096092768344, caller=<optimized out>) at arena.c:181
*3  0x00007f6aa9ad1427 in __GI___vasprintf_chk (result_ptr=0x7ffe17a2ef18, flags=1,
    format=0x401cf8 "STATUS=Ready. %d connections processed.\n", args=0x7ffe17a2ef20) at vasprintf_chk.c:48
*4  0x00007f6aa9d95428 in sd_notifyf () from /lib/x86_64-linux-gnu/libsystemd.so.0
*5  <signal handler called>
*6  0x00007f6aa9a51792 in ptmalloc_unlock_all () at arena.c:270
*7  0x00007f6aa9a931af in __libc_fork () at ../nptl/sysdeps/unix/sysv/linux/x86_64/../fork.c:223
*8  0x0000000000401bba in server_loop ()
*9  0x0000000000401201 in main ()

Use systemd notify system to start daemon and give status.

Various fixes

Use splice instead of copy.

- SO_REUSEADDR wasn't set most of the time since it was depending
  on uninitialized optval.

- BUF_SIZE was less than MTU resulting in sub-optimal performance, 16384
  is even more than enough to accomodate Jumbo Frames.
- Sockets weren't closed in the event of a failure to connect to remote.
- Fixed some compiler warnings about invalid data types.