BUG/MINOR: cli: do not reject the "/<fd>" form of "show fd"

BUG/MINOR: cli: do not reject the "/<fd>" form of "show fd"

Commit da554b7ef added the tgid/fd argument form, where tgid is
optional, and a missing tgid would mean "in each thread group".
However the code considered everything that was not a digit a filter
type, and as / was not a valid filter type, we'd just get an output of
"Invalid FD type".
Fix that by not considering / a filter character.

This must be backported to 3.4.