api: list information about all channels/jobs.

Fire autocmd when channel opens or its info changes.
Add a way for API clients can describe themselves.

src/nvim/terminal.c

@@ -619,6 +619,11 @@ void terminal_get_line_attributes(Terminal *term, win_T *wp, int linenr,
   }
 }
 
+Buffer terminal_buf(const Terminal *term)
+{
+  return term->buf_handle;
+}
+
 // }}}
 // libvterm callbacks {{{