Fixes#534.
* bin/common_file.hh, bin/common_file.cc: Make it possible
to reopen a closed file.
* bin/common_output.cc, bin/common_aoutput.cc: Add a heuristic
to decide when to close files.
* tests/core/serial.test: Add a test case.
* NEWS: Mention the issue.
* bin/common_file.cc, bin/common_file.hh (output_file): Add a
force_append option.
* bin/ltlsynt.cc: Implement the --from-pgame option, and
fix suppot for --csv when multiple inputs are processed.
* NEWS: Mention the new option.
* tests/core/syfco.test: Add a test case.
* tests/core/ltlsynt-pgame.test: New file.
* tests/Makefile.am: Add it.
