Rename ExtendedBufferWhat in 16?

Hi

Commit 31966b15 invented a way for functions dealing with relation
extension to accept a Relation in online code and an SMgrRelation in
recovery code (instead of using the earlier FakeRelcacheEntry
concept).  It seems highly likely that future new bufmgr.c interfaces
will face the same problem, and need to do something similar.  Let's
generalise the names so that each interface doesn't have to re-invent
the wheel?  ExtendedBufferWhat is also just not a beautiful name.  How
about BufferedObjectSelector?  That name leads to macros BOS_SMGR()
and BOS_REL().  Could also be BufMgrObject/BMO, ... etc etc.

This is from a patch-set that I'm about to propose for 17, which needs
one of these too, hence desire to generalise.  But if we rename them
in 17, then AM authors, who are likely to discover and make use of
this interface, would have to use different names for 16 and 17.