config: add new limit: pending_fd_timeout

config: add new limit: pending_fd_timeout

This is one of four commits needed to address CVE-2014-3637.

When a file descriptor is passed to dbus-daemon, the associated D-Bus message
might not be fully sent to dbus-daemon yet. Dbus-daemon keeps the file
descriptor in the DBusMessageLoader of the connection, waiting for the rest of
the message. If the client stops sending the remaining bytes, dbus-daemon will
wait forever and keep that file descriptor.

This patch adds pending_fd_timeout (milliseconds) in the configuration to
disconnect a connection after a timeout when a file descriptor was sent but not
the remaining message.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=80559
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>