Store human-readable information about SSL certificates in the auth store.

This info will later be displayed by 'svn auth', which currently parses
cached SSL certificates to obtain the same information. This new information
can also be accessed by third party clients more easily.

Suggested by: rhuijben

* subversion/include/svn_config.h
  (SVN_CONFIG_AUTHN_HOSTNAME_KEY,
   SVN_CONFIG_AUTHN_FINGERPRINT_KEY,
   SVN_CONFIG_AUTHN_VALID_FROM_KEY,
   SVN_CONFIG_AUTHN_VALID_UNTIL_KEY,
   SVN_CONFIG_AUTHN_ISSUER_DN_KEY): New hash key contants. Adjust the docstring
    for this group of constants which implied that all contants defined here
    were already present before 1.9.

* subversion/libsvn_subr/ssl_server_trust_providers.c
  (ssl_server_trust_file_first_credentials): When reading a cert hash which
   lacks the new human-readable info, add the info and save the cert.
   The idea is to update existing data if possible. However, in practice
   this function will only be called if verification of a cached cert
   suddenly fails. So in most cases only newly saved certs will have
   human-readable information.
  (ssl_server_trust_file_save_credentials): Save new human-readable cert info.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1573625 13f79535-47bb-0310-9956-ffa450edef68