* src/atoms.in: Add hmac-sha2-256, hmac-sha2-512 (RFC 6680),
delete hmac-md5.
* src/algorithms.c (all_symmetric_algorithms): Add hmac-sha2-256
and hmac-sha2-512. Delete hmac-md5.
(default_mac_algorithms): Add hmac-sha2-256 (preferred) and
hmac-sha2-512. Delete hmac-md5.
(lookup_mac): Update recognized mac options. Add "hmac-sha256",
"sha256", "hmac-sha512", "sha512", keep "hmac-sha1", "sha1",
delete "sha", "hmac-sha", "hmac-md5", "md5"
* src/ssh.h (SSH_MAX_MAC_SIZE): Increase to 64, for sha512.
* src/lsh-transport.c (read_user_key): Delete hmac-md5, replaced with
hmac-sha2-256, hmac-sha2-512, for spki_pkcs5_decrypt.
* src/lsh-decrypt-key.c (main): Likewise.