Function to get current dtls retransmit timeout

[Sean Buckheister]

Regarding gnutls_dtls_get_timeout. I updates my stress test to use it
when polling nonblocking sockets, and I found a bug. Apparently, any
gnutls operation that can update the retransmit timer of a nonblocking
socket will do so when executed, regardless of the current timer state.
For example, initiating a handshake and calling gnutls_handshake() every
10ms will double the current retransmit timer on every call.

I'm trying to fix it right now, but first attempts have, again, proven
unsuccessful.