If a file included via <includedir/> is invalid, syslog and skip it

If a file included via <includedir/> is invalid, syslog and skip it

This is more graceful than failing entirely (our previous behaviour),
but more visible than ignoring it completely (the previous behaviour
patched in by Debian and derivatives).

Based on a patch from Daniel Silverstone back in 2004, which was meant
to be temporary; I think it makes sense to change this permanently,
since files in *.d are typically supplied by other packages, whose bugs
shouldn't be able to bring down dbus-daemon.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=19186
Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=230231
Reviewed-by: Colin Walters <walters@verbum.org>