Description:
------------
The "u" specifier is documented as "microseconds" but is interpreted as "fraction of a second".

While this is not the correct behaviour, it is potentially a desirable behaviour, specifically one might expect to be able to interpret a date like `12:13:14.42` as having a fractional part.

I propose adding an "f" specifier (currently unused) which retains this old behaviour, and fixing "u" to work as documented.

Test script:
---------------
echo DateTime::createFromFormat('u', '42')->format('u');

Expected result:
----------------
000042

Actual result:
--------------
420000