Add the try_tx() function.

Add the try_tx() function.

Currently, the TX path (starting from send_packet()) in tinc has three
responsabilities:

 - Making sure packets can be sent (e.g. fetching SPTPS keys);
 - Making sure they can be sent optimally (e.g. fetching non-SPTPS keys
   so that UDP can be used);
 - Sending the actual packet, if feasible.

The first two are closely related; the third one, however, can be
cleanly separated from the other two - meaning, we can loosen code
coupling between sending packets and "optimizing" the way packets are
sent. This will become increasingly important as future commits will
move more tunnel establishment and maintenance code into the TX path,
so we will benefit from a cleaner separation of concerns.

This is especially relevant because of the dual nature of the TX path
(SPTPS versus non-SPTPS), which can make things really complicated when
trying to share low-level code between both.

In this commit, code related to establishing or improving tunnels is
moved away from the core TX path by introducing the "try_*()" family of
function, of which try_sptps() already existed before this commit.

This is a pure refactoring; this commit shouldn't introduce any change
in behavior.