timesyncd: the IP_TOS sockopt is really just an optimization, we shouldn't fail if we can't set it

This partially undos 2f905e821e0342c36f5a5d3a51d53aabccc800bd
calendar: make freeing a calendar spec object deal fine with NULL

In order to make object destruction easier (in particular in combination
with _cleanup_) we usually make destructors deal with NULL objects as
NOPs. Change the calendar spec destructor to follow the same scheme.
delta: use wait_for_terminate_and_warn() to generate warnin when diff fails
NEWS: more preparations for 217
update TODO
time: also support 'infinity' syntax in parse_nsec()

Let's make parse_usec() and parse_nsec() work similar
time: earlier exit from format_timestamp_relative() on special times
time: minor simplification
time: don't do comparison twice