MINOR: cli: improve forward compatibility for show fd

MINOR: cli: improve forward compatibility for show fd

The "<tgid>/" and "/" wildcard forms previously produced no output.
This isn't a bug since they are new, but a script written for future
versions (where the slash form will gain per-thread-group semantics)
would not work the same on 3.4. Make them produce output by dropping
the redundant ctx->fd = -1 wildcard sentinel; also tighten tgid
validation to reject values <= 0.