Corrected minor issues in assert

Assert_ext additionally shows errno and it's meaning. Same as
assert_ret.
Timestamps in different formats supported.

Any time-stamp parsable by strptime accepted. New flag -F handles
format-string. Together with -R & -r, any imaginable clear-text log should be
possible to replay as it originally appeared.

Known issues:
* Parsing of time-stamps containing fractions of seconds are fixed to uS and
must occur last in the ts-field (almost never a ploblem as that is the most
common case).

* Missing now is only buffer-simulation and this little hack should be
done.