sshd: implement ssh config PubkeyAcceptedAlgorithms

sshd: implement ssh config PubkeyAcceptedAlgorithms

Apache MINA sshd 2.6.0 appears to use only the first appropriate
public key signature algorithm for a particular key. See [1]. For
RSA keys, that is rsa-sha2-512. This breaks authentication at servers
that only know the older (and deprecated) ssh-rsa algorithm.

With PubkeyAcceptedAlgorithms, users can re-order algorithms in
the ssh config file per host, if needed. Setting

  PubkeyAcceptedAlgorithms ^ssh-rsa

will put "ssh-rsa" at the front of the list of algorithms, and then
authentication at such servers with RSA keys works again.

[1] https://issues.apache.org/jira/browse/SSHD-1105

Bug: 572056
Change-Id: I86c3b93f05960c68936e80642965815926bb2532
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>