Modern OpenSSH servers reject ssh-rsa (SHA-1) signatures.
Import SHA-2 support to libsec from Plan 9, teach factotum
to sign with SHA256 and SHA512, and handle the
SSH_AGENT_RSA_SHA2_256/512 flags in ssh-agent.
Strip role and hash from factotum RSA key lookups so a
single key can serve both sign and decrypt operations.
Increase _attrfmt buffer from 1024 to 8192 so large RSA
key attributes are not truncated during factotum ctl reads.
The following files were imported from Plan 9 :
- src/libsec/port/sha2block64.c
- src/libsec/port/sha2block128.c
- src/libsec/port/sha2_64.c
- src/libsec/port/sha2_128.c
