Forward-porting from Zope 2.7.

The checkFullTimeStamp() test was sensitive to unique mktime() behavior
on FreeBSD.  See:

http://lists.freebsd.org/pipermail/freebsd-standards/2003-November/000268.html 

The purpose of this test is to exercise ZODB's TimeStamp object, so got
rid of dependence on platform mktime() and time.timezone quirks --
TimeStamp works in GMT, so how mktime() treats tm_isdst should be
irrelevant in all TimeStamp tests.

Also added a comment about the highly non-obvious numeric characteristics
of TimeStamp's treatment of seconds (round-tripping is surprisingly
inaccurate, but for a real reason).