ltlfilt: add a --max-count/-n option

This fixes #44.

* src/bin/ltlfilt.cc: Implement -n/--max-count.
* doc/org/ltlfilt.org, src/tgbatest/randpsl.test: Use it
* NEWS: Document it.

src/tgbatest/randpsl.test

@@ -27,6 +27,8 @@ set -e
 # formulae, and that have a size of at lease 12.
 ../../bin/randltl -n -1 --tree-size 30 --seed 0 --psl a b c |
 ../../bin/ltlfilt -r --size-min 12 --unique |
-../../bin/ltlfilt -v --ltl | head -n 50 |
+../../bin/ltlfilt -v --ltl -n 50 | tee formulas |
 ../../bin/ltlcross '../ltl2tgba -R3 -t %f >%T' '../ltl2tgba -x -R3 -t %f >%T' \
   -F - -f '{{(p1)}[*]:{(p3) && {{!(p1)} xor {!(p3)}}}}'
+
+test `wc -l < formulas` = 50