freopen functionality for golang's io.Writers (library)

 Makes a standard os.File a "reopenable writer" and allows SIGHUP signals to
 reopen log files, as needed by logrotated. This is inspired by the C/Posix
 freopen.
 .
 The simple version reopen.NewFileWriter does unbuffered writing. A call
 to .Reopen closes the existing file handle, and then re-opens it using
 the original filename.
 .
 The more advanced version reopen.NewBufferedFileWriter buffers input and
 flushes when the internal buffer is full (with care) or if 30 seconds
 has elapsed.