MINOR: cli: Make "show fd" aware of per-thread-group FD tables

MINOR: cli: Make "show fd" aware of per-thread-group FD tables

The <tgid>/<fd> argument forms of "show fd" were parsed but the tgid was
ignored so far, the dump always reading the local thread's FD table.
Now that each thread group can have its own table, give the tgid its
intended meaning: a specific tgid dumps that group's table only, while
the wildcard and unspecified forms visit every group in turn, so that
"show fd /<fd>" shows what each group knows about a given FD number.
Each entry's group remains visible in the gid= field. The polled_mask
words are read from the dumped group's slice as well, and the dump can
resume from the right group when the output buffer fills up.
When the tables are shared, all the groups alias the same table, so a
single pass is performed as before and the tgid makes no difference.
The tgid is now also validated against the number of configured thread
groups rather than the maximum supported.