knownhosts: Introduced ssh_known_hosts_get_algorithms_names()

The added internal function obtain a newly allocated string containing a
list of the signature types that can be generated by the keys present in
the known_hosts files, separated by commas.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>