ui: last exec time was wrong for never executed tasks when reloading config
ui: added a mean to translate tasks return code into labels

to add label to numerical return codes, tasks should have such
parameters:
(param return.code.2.label "i/o error")
(param return.code.404.label "not found")
ui: added low water mark resources × hosts view
alerts: stop rules no longer need channels

previously an alert rule with (stop) child needed to declare one real channel
subscription in addition to the (stop), now they can declare 0, or 1, or
several
alert: no longer reminding emited but not raised alerts

fixing a bug introduced by recent commit c8ae40916817
alerts: fixed typos in html alert mail text
examples: restored test configuration for cssoverload
ui: fixed info icon caption and added it to task-related alerts