pgsql: Change the PageGetContents() macro to guarantee its result is

Log Message:
-----------
Change the PageGetContents() macro to guarantee its result is maxalign'd,
thereby forestalling any problems with alignment of the data structure placed
there.  Since SizeOfPageHeaderData is maxalign'd anyway in 8.3 and HEAD, this
does not actually change anything right now, but it is foreseeable that the
header size will change again someday.  I had to fix a couple of places that
were assuming that the content offset is just SizeOfPageHeaderData rather than
MAXALIGN(SizeOfPageHeaderData).  Per discussion of Zdenek's page-macros patch.

Modified Files:
--------------
    pgsql/src/backend/storage/page:
        bufpage.c (r1.79 -> r1.80)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/page/bufpage.c?r1=1.79&r2=1.80)
    pgsql/src/include/access:
        gin.h (r1.23 -> r1.24)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gin.h?r1=1.23&r2=1.24)
    pgsql/src/include/storage:
        bufpage.h (r1.81 -> r1.82)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufpage.h?r1=1.81&r2=1.82)