The usage of flag_std_form needs a thorough cleanup. Additionally, mtime
might want to adopt a workflow in which arguments are normalized at
first and computations are done only after this. This simplify a lot of
cases.

This commit fixes icon/icon#9450 .

for the assignment operator (with a lengthy todo note).

* + timedelta incorrectly used descending, - timedelta ascending iteration
  through leap years.

[divideTwoDatetimeDiffs] bugfixes in src/libmtime.f90, include/mtime_timedelta.h and src/mtime_timedelta.c

Fixes comparison of duration strings with more than 59 seconds, timedeltaToString and according tests.

Fix: mtime - Bug #5037. Type int too small in recently introduced numberOf400YearPeriods. Changing to int64_t

years.

Changing while to do-while to save un-necessary initialisation as suggested by Moritz. Ref commit c22f7403