[sepgsql 3/3] Add db_procedure:execute permission checks

This patch adds sepgsql support for permission checks almost
equivalent to the existing FUNCTION EXECUTE privilege.

This feature is constructed on new OAT_FUNCTION_EXEC event
type being invoked around pg_proc_aclcheck() except for cases
when function's permissions are checked during CREATE or
ALTER commands. (Extension can handle these cases on
OAT_POST_CREATE or OAT_POST_ALTER hooks if needed.)

This patch assumes db_schema:{search} patch is applied on top.
So, please also check the patches below...
https://commitfest.postgresql.org/action/patch_view?id=1003
https://commitfest.postgresql.org/action/patch_view?id=1065

Thanks,
--
KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>