gnutls: loosen the semantics of "use-ssl3"

If SSL 3.0 is disabled, then make "use-ssl3" mean "use the lowest
available TLS version" instead, so that, eg, TLS 1.2 -> TLS 1.0
fallback is still possible.

https://bugzilla.gnome.org/show_bug.cgi?id=738633
gnutls: update default priority string and fallback rules

Change the default "priority" string from "NORMAL:%COMPAT" to
"NORMAL:%COMPAT:%LATEST_RECORD_VERSION". (Correctly-implemented
servers won't care either way, but apparently some servers have been
incorrectly updated for POODLE-safeness by rejecting all handshakes
with SSL 3.0 in the version field [which had long been recommended for
compatibility reasons], even if the handshake offered to negotiate TLS
versions too.)

Change the fallback/"ssl3" priority code so that it never includes
%LATEST_RECORD_VERSION, but does include %COMPAT even if the default
priority doesn't (eg, because it was overridden with
G_TLS_GNUTLS_PRIORITY).

https://bugzilla.gnome.org/show_bug.cgi?id=740087