Do the time calculations with 64-bit integers, rather than in floating

Do the time calculations with 64-bit integers, rather than in floating
point, so we don't have issues with numbers not being exactly
representable; that makes it less likely that the change described below
will change a time stamp if it's not fixing the time stamp (i.e., if
time_day is 0).

The Sniffer manual "Expert Sniffer(R) Network Analyzer Operations,
Release 5.50" says that a frame2_rec has a time stamp with an 8-bit
time_high field and an 8-bit time_day field.  Interpreting the time
stamp that way fixes the time stamps in at least some captures; see, for
example, bug 2251.

Fix/update some comments (for example, the Sniffer documentation is no
longer at that URL).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@24296 f5534014-38df-0310-8fa8-9805f1628bb7