- Claes Jakobsson improved the support for client certificates handling

- Claes Jakobsson improved the support for client certificates handling
  in NSS-powered libcurl. Now the client certificates can be selected
  automatically by a NSS built-in hook. Additionally pre-login to all PKCS11
  slots is no more performed. It used to cause problems with HW tokens.

- Fixed reference counting for NSS client certificates. Now the PEM reader
  module should be always properly unloaded on Curl_nss_cleanup(). If the unload
  fails though, libcurl will try to reuse the already loaded instance.