bin: allow %l to be used to print serial numbers

* NEWS: Mention it.
* bin/autfilt.cc, bin/common_aoutput.cc, bin/common_aoutput.hh,
bin/common_output.cc, bin/common_output.hh, bin/dstar2tgba.cc,
bin/genaut.cc, bin/genltl.cc, bin/ltl2tgba.cc, bin/ltldo.cc,
bin/ltlfilt.cc, bin/ltlgrind.cc, bin/randaut.cc, bin/randltl.cc:
Implement it.
* doc/org/oaut.org: Add a short example.
* tests/core/serial.test: New file.
* tests/Makefile.am: Add it.

NEWS

@@ -1,5 +1,17 @@
 New in spot 2.11.5.dev  (not yet released)
 
+  Command-line tools:
+
+  - In places that accept format strings with '%' sequences, like
+    options --stats, --name, or --output, the new '%l' can now be used
+    to produce the 0-based serial number of the produced object.  This
+    differs from the existing '%L' that is usually related to the line
+    number of the input (when that makes sense).  For instance to
+    split a file that contains many automaton into several files, one
+    per automata, do
+
+        autfilt input.hoa -o output-%l.hoa
+
   Library:
 
   - The following new trivial simplifications have been implemented for SEREs: