distcheck does not allow writes into the build directory, so we
can't use ${testdir}.

Prove that next_check() really runs when there is an error.

There was much more busy-wait looping than I expected.

There are too many troubling possibilities for races otherwise.

This also eliminates the arbitrary sleep time in the constructor.

This falls out automatically from the use of POSIX timers, by
setting the interval to 0.

Whether or not the client nevertheless sees changes in the file
is platform-dependent, since mmap(2) leaves this unspecified for
MAP_PRIVATE. On Linux, changes are seen, and the mapping is
retained after the file is deleted (until munmap).

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.