Add support for SSPI on Windows, which offers NTLM and Negotiate
authentication.

Add functions to use convert a string with length, instead of assuming
NUL termination.

In addition, move the utf8 to 16 conversion routines into the `git_utf8`
namespace instead of using namespaceless `git__` prefixed names.

Name the GSSAPI and ntlmclient authentication providers as such. Today
they're named after the authentication mechanism ("Negotiate", "NTLM")
instead of their implementation.

If we have competing implementations for the same mechanism (eg, a
future Windows SSPI-based provider for Negotiate and NTLM) then this
will get confusing.

Provide a stream interface for Schannel - the native crypto APIs - on
Windows. This allows Windows to use the same HTTP transport that all the
other platforms use, with its own native crypto.

Ultimately this allows us to deprecate WinHTTP and we need not add
support for our socket changes in two places (our HTTP stack and the
WinHTTP stack).

pack: use 64 bits for the number of objects

It is a bit too hidden why we want 64 bits when we're assigning to a 32-bit
integer later on to store the number of objects, so go back to uint32_t and cast
to uint64_t during the size calculation.

Keeping it as a 32-bit value means the min size calculation overflows or gets
truncated which can lead to issues with large packfiles.

cmake: don't include `include/git2`

Including the `include/git2` build path is a seemingly unnecessary
oversight to include the generated `experimental.h` file.