fmttime() <time_shift> argument should be more flexible

1. Currently the only way to specify time_shift in fmttime() is to use expressions like -1y, -2M etc. It's not enough if I want to refer to dates like a month before the previous month 5 years ago. Instead of writing fmttime(%B,-62M) I should be able to write: fmttime(%B,-5y-2M) or fmttime(%B,-5y2M).

2. fmttime() has been introduced to facilitate time formatting in the events for the new trend functions. But there is no way to refer to beginning and/or the end of periods used by trend*() functions. For example, if I calculate trendavg(1M,now/M+1M) I have no way to display end date of now/M+1M in the event name.