fopen and freopen in an append mode gave the C runtime a descriptor with
_O_APPEND, which seeks to the end of the file and then writes. Streams on
separate descriptors overwrote each other's data. FreeBSD opens these
streams with O_APPEND, so every flush lands at the end of the file.
Open append streams with FILE_APPEND_DATA but without FILE_WRITE_DATA,
so Windows writes each flush at the end of the file. ftruncate and
flock reopen the file to get the access such a handle lacks.
