pgsql: Allow ALTER FUNCTION to change a function's strictness,

Log Message:
-----------
Allow ALTER FUNCTION to change a function's strictness, volatility, and
whether or not it is a security definer. Changing a function's strictness
is required by SQL2003, and the other capabilities make sense. Also, allow
an optional RESTRICT noise word to be specified, for SQL conformance.

Some trivial regression tests added and the documentation has been
updated.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        alter_function.sgml (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_function.sgml.diff?r1=1.5&r2=1.6)
        alter_index.sgml (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_index.sgml.diff?r1=1.4&r2=1.5)
    pgsql/src/backend/commands:
        functioncmds.c (r1.55 -> r1.56)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/functioncmds.c.diff?r1=1.55&r2=1.56)
    pgsql/src/backend/nodes:
        copyfuncs.c (r1.297 -> r1.298)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c.diff?r1=1.297&r2=1.298)
        equalfuncs.c (r1.236 -> r1.237)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c.diff?r1=1.236&r2=1.237)
    pgsql/src/backend/parser:
        gram.y (r2.483 -> r2.484)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.483&r2=2.484)
    pgsql/src/backend/tcop:
        utility.c (r1.233 -> r1.234)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c.diff?r1=1.233&r2=1.234)
    pgsql/src/include/commands:
        defrem.h (r1.62 -> r1.63)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/defrem.h.diff?r1=1.62&r2=1.63)
    pgsql/src/include/nodes:
        nodes.h (r1.163 -> r1.164)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/nodes.h.diff?r1=1.163&r2=1.164)
        parsenodes.h (r1.273 -> r1.274)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h.diff?r1=1.273&r2=1.274)
    pgsql/src/test/regress/expected:
        alter_table.out (r1.86 -> r1.87)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/alter_table.out.diff?r1=1.86&r2=1.87)
    pgsql/src/test/regress/sql:
        alter_table.sql (r1.48 -> r1.49)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/alter_table.sql.diff?r1=1.48&r2=1.49)