pgsql: Improve gist XLOG code to follow the coding rules needed to

Log Message:
-----------
Improve gist XLOG code to follow the coding rules needed to prevent
torn-page problems. This introduces some issues of its own, mainly
that there are now some critical sections of unreasonably broad scope,
but it's a step forward anyway. Further cleanup will require some
code refactoring that I'd prefer to get Oleg and Teodor involved in.

Modified Files:
--------------
pgsql/src/backend/access/gist:
gist.c (r1.129 -> r1.130)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gist.c.diff?r1=1.129&r2=1.130)
gistvacuum.c (r1.16 -> r1.17)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistvacuum.c.diff?r1=1.16&r2=1.17)
gistxlog.c (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistxlog.c.diff?r1=1.12&r2=1.13)
pgsql/src/include/access:
gist_private.h (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gist_private.h.diff?r1=1.11&r2=1.12)