Rename PubkeyAcceptedKeyTypes keyword to PubkeyAcceptedAlgorithms.

Rename PubkeyAcceptedKeyTypes keyword to PubkeyAcceptedAlgorithms.
While the two were originally equivalent, this actually specifies the
signature algorithms that are accepted.  Some key types (eg RSA) can be
used by multiple algorithms (eg ssh-rsa, rsa-sha2-512) so the old name is
becoming increasingly misleading.  The old name is retained as an alias.
Prompted by bz#3253, help & ok djm@, man page help jmc@