nss: use a better API for controlling SSL version

This change introduces a dependency on NSS 3.14+.
nss: put SSL version selection into separate fnc
nss: allow to use TLS > 1.0 if built against recent NSS

Bug: http://curl.haxx.se/mail/lib-2013-11/0162.html
nss: unconditionally require NSS_InitContext()

... since we depend on NSS 3.14+ because of SSL_VersionRangeSet() anyway