pgsql: Add pgmagic header block to store compile-time constants: It now

Log Message:
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Add pgmagic header block to store compile-time constants:

It now only checks four things:

Major version number (7.4 or 8.1 for example)
NAMEDATALEN
FUNC_MAX_ARGS
INDEX_MAX_KEYS

The three constants were chosen because:

1. We document them in the config page in the docs
2. We mark them as changable in pg_config_manual.h
3. Changing any of these will break some of the more popular modules:

FUNC_MAX_ARGS changes fmgr interface, every module uses this NAMEDATALEN
changes syscache interface, every PL as well as tsearch uses this
INDEX_MAX_KEYS breaks tsearch and anything using GiST.

Martijn van Oosterhout

Modified Files:
--------------
pgsql/doc/src/sgml:
xfunc.sgml (r1.112 -> r1.113)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.112&r2=1.113)
pgsql/src/backend/utils/fmgr:
dfmgr.c (r1.82 -> r1.83)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/dfmgr.c.diff?r1=1.82&r2=1.83)
pgsql/src/test/regress:
regress.c (r1.65 -> r1.66)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/regress.c.diff?r1=1.65&r2=1.66)

Added Files:
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pgsql/src/include:
pgmagic.h (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgmagic.h?rev=1.1&content-type=text/x-cvsweb-markup)