add more struct timespec/Time formatters

add more struct timespec/Time formatters

Add formatters for durations:

* format_sec_msec: ``1s 337ms'', and
* format_min_sec_msec: ``1m3.370s'' (following time(1)).

Also only use string option types when there is actually an error
path under our control (e. g. with snprintf(3)). (NB the
omnipresent to_string() does not propagate formatting errors
upward so the wrappers we have for it return a plain string.)