Install infrastructure for shared-memory free space map. Doesn't actually

Install infrastructure for shared-memory free space map.  Doesn't actually
do anything yet, but it has the necessary connections to initialization
and so forth.  Make some gestures towards allowing number of blocks in
a relation to be BlockNumber, ie, unsigned int, rather than signed int.
(I doubt I got all the places that are sloppy about it, yet.)  On the
way, replace the hardwired NLOCKS_PER_XACT fudge factor with a GUC
variable.