There are inputs to mktime() where the behavior is not specified and there's
also no single obviously correct behavior. In particular, this affects how
auto-detection of whether DST is in effect is done when tm_isdst = -1 is set
and the time specified does not exist at all or exists twice on that day.
If different implementations are used within an Icinga 2 cluster, that can lead
to inconsistent behavior because different nodes may interpret the same
TimePeriod differently.
This commit introduces a wrapper to mktime(), namely Utility::NormalizeTm()
that implements the behavior provided by glibc. The choice for glibc's behavior
is pretty arbitrary, it was simply picked because most systems that are
officially/fully supported use it (with the only exception being Windows), so
this should give the least possible amount of user-visible changes.
As part of this commit, the closely related helper function mktime_const() is
also moved to Utility::TmToTimestamp() and made a wrapper around the newly
introduced NormalizeTm().
