Change the behaviour of `-v' so that, e.g., stepping a month back

on March 31 won't take you to March 2 or 3 (now the result will
be the last day of February.)

In general, now stepping by months from the last days of the current
month A will take you to the very last day of the target month B if
B is shorter than A.

The previous version would just step to March 31 and rely on mktime(3)
to correct the date.  Despite its simplicity, such way was counter-intuitive
to users and caused pain to shell script writers.

Noticed by:	Igor Timkin <ivt at gamma dot ru>
Approved by:	brian
MFC after:	2 weeks

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