added more on user certificate handling for dovecot

- ssl_verify_depth: will check the maximal certificate chain depth

- ssl_cert_md_algorithm: will check the corresponding certificate fingerprint algorithm (md5/sha1/...)

- cert_loginname: will handle the loginname included in special client certificates (x509 fields)

- cert_fingerprint: allows to access the fingerprint of a certificate inbound of the dovecot (used for select and compare with LDAP backend where the fingerprint of a user is stored)

This patches are ported to dovecot-2.2.x from a patch serie done on Dovecot 2.0.16 done in 2011/2012
In the used LDAP Server (Oracle OID), fingerprints can't be stored as normal
string ... they are encoded BASE64 ... so this are the functions / code parts
to take the fingerprint from client certificate and compare it with the stored
BASE64 encoded string in the LDAP server.

Patch ported from 2.0.16 to 2.2.x