And improve the error message a bit.

Since we mmap length st_size + 1, and since mmap(2) fills the mapped
page past the length of the file with nulls, we ensure that there
is a terminating null after the file contents.

This means that the file can be opened with O_RDONLY and mmap'd
with PROT_READ. So the child process only needs read permissions
on the file.

Pointed out by Nils

And test reading updated file contents after the interval elapses.