Fixup git_attr_value's comment to be recognised as documentation, and
include the definitions needed for clang to parse reset.h such that
it shows up in the documentation.

This fixes #2430.

Maint fixes for ssl initing and ssh exposure

As git_clone now has callbacks to configure the details of the
repository and remote, remove the lower-level functions from the public
API, as they lack some of the logic from git_clone proper.

Analogously to the remote creation callback, provide a way for the user
of git_clone() to create the repository with whichever options they
desire via callback.

This lets a user decide they do want to use keyboard-interactive after
they've compiled.

When linking against libssh2, create the transport.h such that it
contains its definition for custom crypto and keyboard-interactive
callbacks.

If we don't link against libssh2, create an equivalent signature which
has void pointers instead of pointers to libssh2 structures.

This would be one way to fix #2438.

git_remote_set_transport now takes a transport factory rather than a transport
git_clone_options now allows the caller to specify a remote creation callback