yat2m: Implement @item handling with @table item specification.
* doc/yat2m.c (MAX_TABLE_NESTING): New. (table_item_stack): New. (proc_texi_cmd): Add @gcctabopt and @gnupgtabopt. Add handling of @item argument with proc_texi_buffer. Add handling of @table argument, push to table_item_stack.
- GnuPG-bug-id: T6674
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>