Hardwired MAXBACKENDS limit could be history

It would require only very minor changes in the main backend code to
eliminate entirely the hard-wired upper bound MAXBACKENDS. This would
be nice since there'd never be any need to recompile in order to
increase the soft limit MaxBackends (-N). However I see that the
SysV-semaphore emulation code used by the QNX and Darwin ports still
makes use of MAXBACKENDS to size some arrays. I don't especially want
to touch that code, since I am in no position to test it. Perhaps
someone who uses QNX and/or Darwin would like to tweak the sema code
to not depend on a compile-time-constant MAXBACKENDS?

I'm not thinking about getting this done in time for 7.1, but I think
it'd be a nice cleanup for 7.2.

Bruce, a TODO item please:
* Remove compile-time upper limit on number of backends (MAXBACKENDS)

regards, tom lane