GCC C11 warnings

Instead of buf->"typeofbuffer"ReparseBuffer the members will be
referenced with buf->ReparseBuffer."typeofbuffer"

https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_reparse_data_buffer?redirectedfrom=MSDN
calls the union DUMMYUNIONNAME but that looks a bit cluttered.

This adds a `-Wunused-result`-proof `GIT_UNUSED()`, just to demonstrate
that it works. With this, sortedcache.h is now completely
`GIT_WARN_UNUSED_RESULT`-annotated!

This change adds the GIT_WARN_UNUSED_RESULT annotation, which makes the
compiler warn when a return result is not used. This avoids bugs.

Passes w/ gcc 11 on Fedora x64.

Protip: So you don;t have to suffer,

```
perl -pe 's/(-?(?:0x)?[A-Fa-f0-9]+)([Uu])?[Ll][Ll]/\U$2INT64_C(\E$1)/mg'
```

The `git_buf_text` namespace is unnecessary and strange.  Remove it,
just keep the functions prefixed with `git_buf`.

Introduce a function to determine the number of Unicode characters in a
given UTF-8 string.

The `git_blob_filter_options_init` function should be included, to allow
callers in FFI environments to let us initialize an options structure
for them.

As it turns out, the implementation of `git__multiply_int64_overflow` is
full of edge cases and tricky arithmetic. That means that it should have
unit tests.

As a result, a bug in `git__strntol64` was found (and fixed!) in
clang+32-bit.