yat2m: Generate \- for roff for some commands.
* doc/yat2m.c (proc_texi_cmd): New flag to enable generating \- for roff for @command, @code, @url and @option for their argument, and @example and @smallexample until their @end. (proc_texi_buffer): Emit \- when enabled.
- GnuPG-bug-id: T6674
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>