Rearrange handling of MAXBACKENDS a little bit. The default setting

Rearrange handling of MAXBACKENDS a little bit.  The default setting
of MAXBACKENDS is now 1024, since all it's costing is about 32 bytes of memory
per array slot.  configure's --with-maxbackends switch now controls DEF_MAXBACKENDS
which is simply the default value of the postmaster's -N switch.  Thus,
the out-of-the-box configuration will still limit you to 64 backends,
but you can go up to 1024 backends simply by restarting the postmaster with
a different -N switch --- no rebuild required.