pgsql: Fix executor/spi.h to follow our usual conventions for include

Log Message:
-----------
Fix executor/spi.h to follow our usual conventions for include files, ie,
not include postgres.h nor anything else it doesn't directly need.  Add
#includes to calling files as needed to compensate.  Per my proposal of
yesterday.

This should be noted as a source code change in the 8.4 release notes,
since it's likely to require changes in add-on modules.

Modified Files:
--------------
    pgsql/contrib/spi:
        autoinc.c (r1.15 -> r1.16)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/spi/autoinc.c?r1=1.15&r2=1.16)
        insert_username.c (r1.16 -> r1.17)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/spi/insert_username.c?r1=1.16&r2=1.17)
        moddatetime.c (r1.14 -> r1.15)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/spi/moddatetime.c?r1=1.14&r2=1.15)
        refint.c (r1.33 -> r1.34)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/spi/refint.c?r1=1.33&r2=1.34)
        timetravel.c (r1.29 -> r1.30)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/spi/timetravel.c?r1=1.29&r2=1.30)
    pgsql/contrib/tablefunc:
        tablefunc.c (r1.58 -> r1.59)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/tablefunc/tablefunc.c?r1=1.58&r2=1.59)
    pgsql/contrib/xml2:
        xpath.c (r1.21 -> r1.22)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xpath.c?r1=1.21&r2=1.22)
        xslt_proc.c (r1.13 -> r1.14)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/xml2/xslt_proc.c?r1=1.13&r2=1.14)
    pgsql/doc/src/sgml:
        spi.sgml (r1.62 -> r1.63)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/spi.sgml?r1=1.62&r2=1.63)
    pgsql/src/backend/executor:
        spi.c (r1.204 -> r1.205)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c?r1=1.204&r2=1.205)
    pgsql/src/backend/utils/adt:
        ri_triggers.c (r1.111 -> r1.112)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ri_triggers.c?r1=1.111&r2=1.112)
        ruleutils.c (r1.294 -> r1.295)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c?r1=1.294&r2=1.295)
        tsquery_rewrite.c (r1.13 -> r1.14)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsquery_rewrite.c?r1=1.13&r2=1.14)
        tsvector_op.c (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsvector_op.c?r1=1.20&r2=1.21)
        xml.c (r1.82 -> r1.83)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.82&r2=1.83)
    pgsql/src/include/executor:
        spi.h (r1.68 -> r1.69)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/spi.h?r1=1.68&r2=1.69)
    pgsql/src/pl/plperl:
        plperl.c (r1.143 -> r1.144)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.c?r1=1.143&r2=1.144)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.118 -> r1.119)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.118&r2=1.119)
        pl_exec.c (r1.226 -> r1.227)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c?r1=1.226&r2=1.227)
        plpgsql.h (r1.107 -> r1.108)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h?r1=1.107&r2=1.108)
    pgsql/src/pl/tcl:
        pltcl.c (r1.124 -> r1.125)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/tcl/pltcl.c?r1=1.124&r2=1.125)
    pgsql/src/test/regress:
        regress.c (r1.71 -> r1.72)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/regress.c?r1=1.71&r2=1.72)