pgsql: Fix contrib/pageinspect to not create an ABI breakage between 8.3

Log Message:
-----------
Fix contrib/pageinspect to not create an ABI breakage between 8.3 and 8.4.
The original implementation of the 3-argument form of get_raw_page() risked
core dumps if the 8.3 SQL function definition was mistakenly used with the
8.4 module, which is entirely likely after a dump-and-reload upgrade. To
protect 8.4 beta testers against upgrade problems, add a check on PG_NARGS.

In passing, fix missed additions to the uninstall script, and polish the
docs a trifle.

Modified Files:
--------------
pgsql/contrib/pageinspect:
pageinspect.sql.in (r1.6 -> r1.7)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pageinspect/pageinspect.sql.in?r1=1.6&r2=1.7)
rawpage.c (r1.11 -> r1.12)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pageinspect/rawpage.c?r1=1.11&r2=1.12)
uninstall_pageinspect.sql (r1.4 -> r1.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pageinspect/uninstall_pageinspect.sql?r1=1.4&r2=1.5)
pgsql/doc/src/sgml:
pageinspect.sgml (r1.5 -> r1.6)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pageinspect.sgml?r1=1.5&r2=1.6)