Remove unused nagios.sh
Simplify cron.sh

This delegates the task of mutex locking to lockrun, resulting in a much
simpler cron.sh
Simplify and clean up run_feature.sh

Again, remove half-baked locking logic, on the basis that this can be handled
by a custom tool higher up. (See comment about lockrun in cron.sh)
Fix nagios_check_cache.sh

This simplifies and cleans up nagios_check_cache, and also makes its behaviour
slightly less system-specific and fragile.

It also removes the second-guessing "if-it-failed-then-run-it-again" logic.