BUG/MINOR: cli: use the current argument to parse the FD spec in "show fd"

BUG/MINOR: cli: use the current argument to parse the FD spec in "show fd"

Commit 1cb041a6e added filtering to show fd, which means the number of
arguments can now change, however args[2] was still used from time to
time. Fix this by using args|arg] instead when relevant.

Note that before 3.4, there is only one instance of that, atoi(args[2])
should be atoi(args[arg]). That could lead to the wrong fd being used.

This should be backported up to 2.8.