Add new `libssh2_session_callback_set2()` API that deprecates
`libssh2_session_callback_set()`.
The new implementation offers the same functionality, but accepts and
returns a generic function pointer (of type `libssh2_cb_generic *`), as
opposed to the old function that used data pointers (`void *`). The new
solution thus avoids data to function (and vice versa) pointer
conversions, which has undefined behaviour in standard C.
About the name: It seems the `*2` suffix was used in the past for
replacement functions for deprecated ones. Let's stick with that.
`*_ex` was preferred for new functions that extend existing ones with
new features.
Closes#1285
