BufferedFileFdPrivate: Make InternalFlush() save against errors

BufferedFileFdPrivate: Make InternalFlush() save against errors

Previously, if flush errored inside the loop, data could have
already been written to the wrapped descriptor without having
been removed from the buffer.

Also try to work around EINTR here. A better solution might be
to have the individual privates detect an interrupt and return
0 in such a case, instead of relying on errno being untouched
in between the syscall and the return from InternalWrite.