RFC: Change mtree nsec handling?

In looking at interoperability between libarchive's
mtree support and the mtree(8) program, I found
that mtree formats timestamps rather strangely.

For example, a timestamp of 1233295862.000001
(1233295682 seconds and 1000 nanoseconds)
will be printed like this by mtree:
    time=1233295862.1000
Unsurprisingly, the mtree parsing works the same
way in reverse.

Basically, mtree is printing (and reading) the time
as whole seconds and nanoseconds separated by a period,
which 90% of the time is the same as a decimal
floating-point number of seconds, but not the other
10% of the time.  This is not documented in mtree.8.
I was quite surprised by it.

The attached patch changes this to use a more conventional
notation (although the patch could use a little more tweaking
to handle >9 digits after the decimal point).

Any concerns about this?

Tim