remove hmac-ripemd160; ok dtucker

Add FingerprintHash option to control algorithm used for key
fingerprints. Default changes from MD5 to SHA256 and format
from hex to base64.

Feedback and ok naddy@ markus@

forward-declare struct sshbuf so consumers don't need to include sshbuf.h

New key API: refactor key-related functions to be more library-like,
existing API is offered as a set of wrappers.

with and ok markus@

Thanks also to Ben Hawkes, David Tomaschik, Ivan Fratric, Matthew
Dempsky and Ron Bowes for a detailed review a few months ago.

revert __bounded change; it causes way more problems for portable than
it solves; pointed out by dtucker@

use __bounded(...) attribute recently added to sys/cdefs.h instead of
longform __attribute__(__bounded(...));

for brevity and a warning free compilation with llvm/clan

replace openssl HMAC with an implementation based on our ssh_digest_*
ok and feedback djm@

Introduce digest API and use it to perform all hashing operations
rather than calling OpenSSL EVP_Digest* directly. Will make it easier
to build a reduced-feature OpenSSH without OpenSSL in future;
feedback, ok markus@