file writer: Use xwrite() instead of plain write().

file writer: Use xwrite() instead of plain write().

Currently the file writer's ->post_select() sets t->error to -1 on
errors, rather than using a proper error code. This may result in
a segfault when this number is passed to para_strerror().

Replacing the call to write() by xwrite() not only gives a proper
error code but also treats EAGAIN as a non-fatal error.