http_ntlm: add support for NSS

When configured with '--without-ssl --with-nss', NTLM authentication
now uses NSS crypto library for MD5 and DES.  For MD4 we have a local
implementation in that case.  More details are available at
https://bugzilla.redhat.com/603783

In order to get it working, curl_global_init() must be called with
CURL_GLOBAL_SSL or CURL_GLOBAL_ALL.  That's necessary because NSS needs
to be initialized globally and we do so only when the NSS library is
actually required by protocol.  The mentioned call of curl_global_init()
is responsible for creating of the initialization mutex.

There was also slightly changed the NSS initialization scenario, in
particular, loading of the NSS PEM module.  It used to be loaded always
right after the NSS library was initialized.  Now the library is
initialized as soon as any SSL or NTLM is required, while the PEM module
is prevented from being loaded until the SSL is actually required.