Position(). This used up a virtual slot of BPositionIO. GetSize() needed to be
implemented in BFile, which uses the BStatable version as before. This should
hopefully be binary compatible. The only problem could be in source
compatibility when there are classes inheriting from BPositionIO implementing
their own GetSize().
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@17593 a95241bf-73f2-0310-859d-f6bbb57e9c96