doc: understand `firstnumber=last'
doc: understand `firstnumber=last'

	* doc/tex2chk ($previous_label): New.
	(register): Use it to glue together successive listings in a
	single test.

aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi