According to `man open`:

> The file is opened in append mode. Before each write(2), the file offset
is positioned at the end of the file, as if with lseek(2). The modi‐
fication of the file offset and the write operation are performed as a
single atomic step.

Cleanup to allow use of most functions when linking the library, including mounting and creating filesystems

This is enough to fix as. I don't know if it correctly fills the space
with zeros when writing beyond the current EOF.

I didn't notice there were two copies of this code

Prevents potential undefined behavior based on how the call is made. Of
course, eventually these fields should actually be set.