Small amendments in newly merged task runtime limit functionailty.

Better help message in case of invalid -L option value. Better
catching of some exceptions (e.g. Sumday) - even though by using
uncool exception catching at the moment. Some renaming, e.g. using
word pairs (being/end, star/stop), using three letters for week day
abbreviation as is common in OS 'date' commands, using
parse_runtime_limit(), etc.