sshd: handle IdentitiesOnly with an SSH agent

sshd: handle IdentitiesOnly with an SSH agent

If an SSH agent is used but "IdentitiesOnly yes" is set, only those
keys from the agent that correspond to one of the keys explicitly given
via an IdentityFile directive are to be used.

Implement this by filtering the list of keys obtained from the agent
against the list of IdentityFiles, each entry suffixed with ".pub".
Load the public keys from these files, and ignore all other keys from
the agent. Keys without ".pub" file are also ignored.

Apache MINA sshd has no operation to load only the public key from a
private key file, so we have to rely on *.pub files.

Bug: 577053
Change-Id: I75c2c0b3ce35781c933ec2944bd6da1b94f4caf9
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>