MINOR: cli: give the show_fd helpers the ability to report a suspicious entry

MINOR: cli: give the show_fd helpers the ability to report a suspicious entry

Now the show_fd helpers at the transport and mux levels return an integer
which indicates whether or not the inspected entry looks suspicious. When
an entry is reported as suspicious, "show fd" will suffix it with an
exclamation mark ('!') in the dump, that is supposed to help detecting
them.

For now, helpers were adjusted to adapt to the new API but none of them
reports any suspicious entry yet.