Make SIGHUP singal handler equivalent to SIGTERM and SIGINT.

offlineimap has several frontends that encourage running it from a
terminal under an X session. When X session closes for a system
shutdown, the terminals exit, after sending SIGHUP to their children.

Previously SIGHUP was treated to be equivalent to SIGUSR1, i.e. wake
up and sync all accounts. This causes delays during shutdown.

According to Wikipedia [0], SIGHUP has been repurposed from a
historical meaning to one of:
 * re-read configuration files, or reinitialize (e.g. Apache, sendmail)
 * controlling pseudo or virtual terminal has been closed

I believe second meaning is more appropriate for offlineimap, and
hence this patch makes SIGHUP to be handled in the same way SIGTERM
and SIGINT are handled.

This patch also adds documentation of the current handling of
SIGTERM/SIGINT/SIGHUP signals.

[0] http://en.wikipedia.org/wiki/SIGHUP

Debian-Bug: http://bugs.debian.org/670120
Reported-By: Steve Langasek <steve.langasek@canonical.com>
Signed-off-by: Dmitrijs Ledkovs <xnox@debian.org>