exec: Kill child process group to catch unregistered children

Remove kill wrapper that was introduced in 50180cc... because it
doesn't seems to be very useful, after all.
main: Add -a cmdline flag to assure crash reproducibility
gcc: Remove gcc format string warning
main: Make BUNNY.log line-buffered
exec: Improve signal handler correctness

Safe and restore errno and make (some) variables volatile.  Remove
non-reentrant (?) printf() call.

Probably it would make sense to remove timeout() altogether and
integrate time-keeping into the main loop.