time-util: merge format_timestamp_internal() and format_timestamp_internal_us()

time-util: merge format_timestamp_internal() and format_timestamp_internal_us()

The time_util.c provides format_timestamp_internal() and
format_timestamp_internal_us() functions for a timestamp formating. Both
functions are very similar and differ only in formats handling.

We can add additional boolean parameter to the format_timestamp_internal()
function which will represent is a format for us timestamp or not.
This allows us to get rid of format_timestamp_internal_us() that is prevent
code duplication.

We can remove format_timestamp_internal_us() safely, because it is static and
has no users outside of the time_util.c. New fourth parameter will be passed
inside of the format_timestamp(), format_timestamp_us() and etc, functions,
but the public API is not changed.