Handle the 2GiB boundary case of the max filesize autostop condition properly so...

Handle the 2GiB boundary case of the max filesize autostop condition properly so that we avoid overflow conditions and so that we ensure we don't capture more than 2GiB.  Also, document the max filesize autostop value of 2GIB as well as indicating that it's truly GiB and not GB.

This fixes the problem reported on ask: http://ask.wireshark.org/questions/23891/wireshark-wont-run-with-multiple-capture-files

#BACKPORT(1.10) ... not sure about 1.8?

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@51576 f5534014-38df-0310-8fa8-9805f1628bb7