Path MTU discovery implementation

This implementation uses gnutls dtls heartbeat API to test path MTU.
heartbeat allowing messages with automated response in a datagram,
the application is able to guess the MTU via a timeout in the heartbeat.
(timeout on packet sent and no response, implies that the MTU is lower
than the lost payload.)
To minimize false positives (a response is lost for example), each attempt
triggers one retry on the first timeout.
This version ensures a minimal MTU of 512 bytes will be returned in
case of any failure in the procedure.
For retrocompatibility with non heartbeat capable clients,
a fallback MTU is set at 1280.

Change-Id: Ib9a7f63a70e8bdad239d8fc103779a0f2c387e87
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>