Fix assorted defects in 09adc9a8c09c9640de05c7023b27fb83c761e91c.

Fix assorted defects in 09adc9a8c09c9640de05c7023b27fb83c761e91c.

That commit increased all shared memory allocations to the next higher
multiple of PG_CACHE_LINE_SIZE, but it didn't ensure that allocation
started on a cache line boundary.  It also failed to remove a couple
other pieces of now-useless code.

BUFFERALIGN() is perhaps obsolete at this point, and likely should be
removed at some point, too, but that seems like it can be left to a
future cleanup.

Mistakes all pointed out by Andres Freund.  The patch is mine, with
a few extra assertions which I adopted from his version of this fix.