pki: Allow filtering accepted public key types based on the configuration

This effectively allows to disable using the SHA2 extension, disable
other old public key mechanisms out of the box (hello DSA) or force
the new SHA2-based key algorithm types if needed.

This exposes the  default_methods  array from  kex.c.

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/kex.h

@@ -43,6 +43,7 @@ char **ssh_space_tokenize(const char *chain);
 int ssh_get_kex1(ssh_session session);
 char *ssh_find_matching(const char *in_d, const char *what_d);
 const char *ssh_kex_get_supported_method(uint32_t algo);
+const char *ssh_kex_get_default_methods(uint32_t algo);
 const char *ssh_kex_get_description(uint32_t algo);
 
 #endif /* KEX_H_ */

include/libssh/pki.h

@@ -138,4 +138,5 @@ ssh_string ssh_srv_pki_do_sign_sessionid(ssh_session session,
 ssh_public_key ssh_pki_convert_key_to_publickey(const ssh_key key);
 ssh_private_key ssh_pki_convert_key_to_privatekey(const ssh_key key);
 
+int ssh_key_algorithm_allowed(ssh_session session, const char *type);
 #endif /* PKI_H_ */