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.

IPv6 addresses should be used identically internally; we should not
denote them with brackets in one operating system and without them in
another.

This change makes it possible to build with newer versions of gcc
without warnings. There were two warnings issued:

* gcc 8 added
  [`-Wstringop-truncation`](https://developers.redhat.com/blog/2018/05/24/detecting-string-truncation-with-gcc-8/),
  which warns if a call to `strncpy(3)` is prone to accidentally
  truncating the destination string, since `strncpy(3)` does NOT add a
  terminating `NULL` if the destination buffer is not large enough to
  hold the input.

  This change uses the pattern suggested in
  https://us-cert.cisa.gov/bsi/articles/knowledge/coding-practices/strncpy-and-strncat
  to fix the locations flagged by gcc.
* There was a potentially uninitialized access of `dest` in `fs_copy`.

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.

Using RC4 is not a _certificate_ problem, it's a cipher problem.  The
SSL implementation should and will fail with an unrecoverable error
(-1).  There's no opportunity to accept/continue.

This used to fail with an error indicating a mis-use of OpenSSL on platforms
using it due to poor error handling. Re-enable it even if this isn't the right
error code to use for now.

Use a no-allocation approach to the TLS data abstraction.

Clarify the `git_atomic` type and functions now that we have a 64 bit
version as well (`git_atomic64`).