sysdeps: move _dbus_system_log() into the shared library

sysdeps: move _dbus_system_log() into the shared library

This is in preparation for optionally making _dbus_warn() use it.
dbus-daemon closes its stderr under some circumstances, including
when launched by dbus-launch, which makes failures in that
situation rather hard to debug.

_dbus_system_log() is the same on Unix and Windows, so move it
to dbus-sysdeps.c. _dbus_system_logv() remains platform-specific.

Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=97009
[smcv: move the #include for syslog.h, too]
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>