* bin/common_output.cc: Set the output stream for LTL formats.
* tests/core/serial.test: Add a test case about this issue that also
improve the covering of the previous patch about saving file
descriptors.
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.