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.

Use shell's 'test -t' to detect when running a test and strip color codes.

Co-authored-by: Valentin Perrelle <valentin.perrelle@cea.fr>

Viewing Flamegraphs requires some specific Perl modules (e.g. open),
which are not necessarily available even with conf-perl.
Since this feature is optional and cannot be entirely tested via command-line,
removing it for now is a better approach than trying to extensively check that
each dependence is available in the test machine.