gtlsbackend: Default to indicating no support for DTLS in GTlsBackend

I’m unsure what the original reasoning for returning TRUE by default
from supports_dtls was, but it is not backwards-compatible. If a
pre-existing GTlsBackend implementation never implements the
supports_dtls vfunc, the supports_dtls() method will magically return
TRUE rather than FALSE.

Since any backend which does implement DTLS should be implementing the
supports_dtls vfunc (and no DTLS-supporting backends have actually been
merged yet; see bug #697908), it seems safer to make this slight API
break in the name of backwards compatibility than to leave it as
returning TRUE incorrectly.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=787485