pgsql: Allow functions returning void or cstring to appear in FROM

Log Message:
-----------
Allow functions returning void or cstring to appear in FROM clause,
to make life cushy for the JDBC driver.  Centralize the decision-making
that affects this by inventing a get_type_func_class() function, rather
than adding special cases in half a dozen places.

Modified Files:
--------------
    pgsql/src/backend/access/common:
        tupdesc.c (r1.106 -> r1.107)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/tupdesc.c.diff?r1=1.106&r2=1.107)
    pgsql/src/backend/executor:
        nodeFunctionscan.c (r1.27 -> r1.28)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeFunctionscan.c.diff?r1=1.27&r2=1.28)
    pgsql/src/backend/parser:
        parse_relation.c (r1.100 -> r1.101)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_relation.c.diff?r1=1.100&r2=1.101)
    pgsql/src/backend/utils/cache:
        lsyscache.c (r1.116 -> r1.117)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/lsyscache.c.diff?r1=1.116&r2=1.117)
    pgsql/src/include/utils:
        lsyscache.h (r1.90 -> r1.91)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/lsyscache.h.diff?r1=1.90&r2=1.91)