bus signal_handler: don't use _dbus_warn, and don't pretend to be portable

bus signal_handler: don't use _dbus_warn, and don't pretend to be portable

_dbus_warn isn't async-signal-safe, so that's out. We can use write()
instead; it's POSIX but not ISO C, but then again, so are signals.
Accordingly, guard it with DBUS_UNIX.

dbus-sysdeps-util-win doesn't actually implement _dbus_set_signal_handler
anyway, so not compiling this code on non-Unix seems more honest.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=33336
Reviewed-by: Thiago Macieira <thiago@kde.org>