Android NDK does not have a `struct timespec` in its `struct stat`
for nanosecond support, instead it has a single nanosecond member inside
the struct stat itself.  We will use that and use a macro to expand to
the `st_mtim` / `st_mtimespec` definition on other systems (much like
the existing `st_mtime` backcompat definition).

Use the `giterr_set` function, which actually supports `GITERR_OS`.
The `giterr_set_str` function is exposed for external users and will
not append the operating system's error message.

Windows defines `timeval` with `long`, which we cannot
sanely cope with.  Instead, use a custom timeval struct.

Provide functionality to set the time on a filesystem entry,
using utimes or futimes on POSIX type systems or SetFileTime
on Win32.

In order to avoid racy-git, we zero out the file size for entries with
the same timestamp as the index (or during the initial checkout). This
is the case in a couple of crlf tests, as the code is fast enough to do
everything in the same second.

As we know that we do not perform the modification just after writing
out the index, which is what this is designed to work around, tick the
mtime of the index file such that it doesn't agree with the files
anymore, and we do not zero out these entries.

Introduce a new `git_path_diriter` that can iterate directories
efficiently for each platform.

A non-readable mapping of a file causes an access violation in
the pack tests. Always use PROT_READ to work around this.