Store but don't use keys of unsupported types on the known_hosts file.
Currently, when libssh2 parses a known_host file containing keys of some
type it doesn't natively support, it stops reading the file and returns
an error.
That means, that the known_host file can not be safely shared with other
software supporting other key types (i.e. OpenSSH).
This patch adds support for handling keys of unknown type. It can read
and write them, even if they are never going to be matched.
At the source level the patch does the following things:
- add a new unknown key type LIBSSH2_KNOWNHOST_KEY_UNKNOWN
- add a new slot (key_type_name) on the known_host struct that is
used to store the key type in ascii form when it is not supported
- parse correctly known_hosts entries with unknown key types and
populate the key_type_name slot
- print correctly known_hosts entries of unknown type
- when checking a host key ignore keys that do not match the key
Fixes#276
85c6627c86