bin: handle thousands of output files

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.hh

@@ -37,6 +37,8 @@ public:
 
   void close(const std::string& name);
 
+  void reopen_for_append(const std::string& name);
+
   bool append() const
   {
     return append_;