We create a lockfile to update files under GIT_DIR. Sometimes these
files are actually located elsewhere and a symlink takes their place. In
that case we should lock and update the file at its final location
rather than overwrite the symlink.

When we have an error renaming the lockfile, we need to make sure
that we remove it upon cleanup. For this, we need to keep track of
whether we opened the file and whether the rename succeeded.

If we did create the lockfile but the rename did not succeed, we
remove the lockfile. This won't protect against all errors, but
the most common ones (target file is open) does get handled.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Use `size_t` to hold the size of arrays to ease overflow checking,
lest we check for overflow of a `size_t` then promptly truncate
by packing the length into a smaller type.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

When renaming a lock file to its final location, we need to make sure
that it is replaced atomically.

We currently have a workaround for Windows by removing the target file.
This means that the target file, which may be a ref or a packfile, may
cease to exist for a short wile, which shold be avoided.

Implement the workaround only in Windows, by making sure that the file
we want to replace is writable.