add tags in buffer created with /exec -nf

[Pascal Poitras Dubois]

Feature description

Since version 3.5, buffer with free content can have tags in line. Since the exec plugin was made before 3.5, when you are using /exec -nf, there is no "exec_stdout" tags nor exec_cmd_<name> tags (when you use /exec -name). I think we should have this now that buffer with free content support tags in lines