Introduce a simple counter that `p_fsync` implements.  This is useful
for ensuring that `p_fsync` is called when we expect it to be, for
example when we have enabled an odb backend to perform `fsync`s when
writing objects.

`snprintf` requires a _format_ but does not require _arguments_ to the
format.  eg: `snprintf(buf, 42, "hi")` is perfectly legal.  Expand the
macro to match.

Without this, `p_sprintf(buf, 42, "hi")` errors with:

```
error: expected expression
                p_snprintf(msg, 42, "hi");
                ^
src/unix/posix.h:53:34: note: expanded from macro 'p_snprintf'
                                 ^
/usr/include/secure/_stdio.h:57:73: note: expanded from macro 'snprintf'
  __builtin___snprintf_chk (str, len, 0, __darwin_obsz(str),
__VA_ARGS__)
```

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

Introduce `git_thread_exit`, which will allow threads to terminate at an
arbitrary time, returning a `void *`.  On Windows, this means that we
need to store the current `git_thread` in TLS, so that we can set its
`return` value when terminating.

We cannot simply use `ExitThread`, since Win32 returns `DWORD`s from
threads; we return `void *`.