pgsql: Support SET FROM CURRENT in CREATE/ALTER FUNCTION, ALTER

Log Message:
-----------
Support SET FROM CURRENT in CREATE/ALTER FUNCTION, ALTER DATABASE, ALTER ROLE.
(Actually, it works as a plain statement too, but I didn't document that
because it seems a bit useless.)  Unify VariableResetStmt with
VariableSetStmt, and clean up some ancient cruft in the representation of
same.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        alter_database.sgml (r1.19 -> r1.20)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_database.sgml?r1=1.19&r2=1.20)
        alter_function.sgml (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_function.sgml?r1=1.14&r2=1.15)
        alter_role.sgml (r1.8 -> r1.9)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_role.sgml?r1=1.8&r2=1.9)
        alter_user.sgml (r1.43 -> r1.44)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_user.sgml?r1=1.43&r2=1.44)
        create_function.sgml (r1.76 -> r1.77)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_function.sgml?r1=1.76&r2=1.77)
    pgsql/src/backend/commands:
        dbcommands.c (r1.197 -> r1.198)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c?r1=1.197&r2=1.198)
        functioncmds.c (r1.84 -> r1.85)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/functioncmds.c?r1=1.84&r2=1.85)
        user.c (r1.176 -> r1.177)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/user.c?r1=1.176&r2=1.177)
    pgsql/src/backend/nodes:
        copyfuncs.c (r1.381 -> r1.382)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c?r1=1.381&r2=1.382)
        equalfuncs.c (r1.312 -> r1.313)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c?r1=1.312&r2=1.313)
    pgsql/src/backend/parser:
        gram.y (r2.601 -> r2.602)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.601&r2=2.602)
    pgsql/src/backend/tcop:
        pquery.c (r1.116 -> r1.117)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/pquery.c?r1=1.116&r2=1.117)
        utility.c (r1.285 -> r1.286)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c?r1=1.285&r2=1.286)
    pgsql/src/backend/utils/misc:
        guc.c (r1.415 -> r1.416)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.415&r2=1.416)
    pgsql/src/include/nodes:
        nodes.h (r1.203 -> r1.204)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/nodes.h?r1=1.203&r2=1.204)
        parsenodes.h (r1.352 -> r1.353)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h?r1=1.352&r2=1.353)
    pgsql/src/include/utils:
        guc.h (r1.84 -> r1.85)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc.h?r1=1.84&r2=1.85)