[Eva] Aborts the analysis cleanly on user interruption sigint (Ctrl-C).

Cleans up and saves partial results on sigint signal.
Signal handlers are now registered at the start of an analysis, and previous
signal handlers are restored at the end.
New function [protect f ~cleanup] in value_util, that runs [f] and applies
[cleanup] in case of a user interruption or a Frama-C error, only if option
-save is set.