ldap: server and client certificate support

This adds 4 more options to the ldap authentication method:

verify: boolean
  If enabled, the server certificate must be valid

capath: path to a file or directory
  The CA to use to verify the server certificate. Used only
  if 'verify' is true.

cert: path to a certificate
  Used as client certificate when connecting to a server,
  provided 'secure' is true. Requires 'certkey' to be set.

certkey: path to the certificate's key
  Required only used when 'cert' is used.