ltlcross: report exit_status and exit_code columns in CSV and JSON

* src/bin/ltlcross.cc: Report exit_status and exit_code columns in CSV
and JSON files.  Also output lines for failed translations, and add
a --omit-missing option to disable that.  Move the time column right
after exit_status and exit_code.
* src/bin/man/ltlcross.x: Document each column of the output.
* bench/ltl2tgba/tools: Use the "{name}cmd" notation.
* bench/ltl2tgba/sum.py: Adjust to the new columns.
* bench/ltl2tgba/README: Update to point to the man page for a
description of the columns.
* bench/ltl2tgba/Makefile.am: Build results.pdf as said announced in
README.
* bench/spin13/html.bottom: Update code to ignore these two new
columns and lines with null values.
* src/tgbatest/ltlcross3.test: Add tests.
* doc/org/ltlcross.org: Adjust examples.
* NEWS: Mention this.

NEWS

@@ -7,7 +7,13 @@ New in spot 1.2a (not released)
          ltlcross '{small} ltl2tgba -s --small %f >%N' ...
       will run the command "ltl2tgba -s --small %f >%N", but only
       print "small" in output files.
-
+    - ltlcross' CSV and JSON output now contains two additional
+      columns: exit_status and exit_code, used to report failures of
+      the translator.  If the translation failed, only the time is
+      reported, and the rest of the statistics, which are missing,
+      area left empty (in CVS) or null (in JSON).  A new option,
+      --omit-missing can be used to remove lines for failed
+      translations, and remove these two columns.
   * Bug fixes:
     - ltlcross' CSV output now stricly follows RFC 4180.
     - ltlcross failed to report missing input or output escape sequences