util: make sure reset_all_signal_handlers() continues with all other signal handlers when one sigaction() fails

After all, we usually don't check for failures here, and it is better to
do as much as we can...
util: reset signals when we fork off agents

If we invoke agents, we should make sure we actually can kill them
again. I mean, it's probably not our job to cleanup the signals if our
tools are invoked in weird contexts, but at least we should make sure,
that the subprocesses we invoke and intend to control work as intended.

Also see:

http://lists.freedesktop.org/archives/systemd-devel/2014-August/022460.html
util: make use of newly added reset_signal_mask() call wherever appropriate