agent: Enable "relax" in the trustlist by default and add "norelax"

* agent/trustlist.c (read_one_trustfile): Enabled the relax flag be
default.  Add new keyword "norelax".

In any real world settings the "relax" flag is always set. Thus it is
better to make it the default and provide a way to disable it. In
fact, it is questionable whether this flag make sense at all. It is
some kind of legacy thing.