MINOR: cli: add a new "show fd" command

MINOR: cli: add a new "show fd" command

This one dumps the fdtab for all active FDs with some quickly interpretable
characters to read the flags (like upper case=set, lower case=unset). It
can probably be improved to report fdupdt[] and/or fdinfo[] but at least it
provides a good start and allows to see how FDs are seen. When the fd owner
is a connection, its flags are also reported as it can help compare with the
polling status, and the target (fe/px/sv) as well. When it's a listener, the
listener's state is reported as well as the frontend it belongs to.