MINOR: cli/show_fd: report some easily detectable suspicious states

MINOR: cli/show_fd: report some easily detectable suspicious states

A file descriptor which maps to a connection but has more than one
thread in its mask, or an FD handle that doesn't correspond to the FD,
or wiht no mux context, or an FD with no thread in its mask, or with
more than 1 million events is flagged as suspicious.

(cherry picked from commit dacfde4ba42e3a13dd5b13b7df7a37db06d702a6)
Signed-off-by: Willy Tarreau <w@1wt.eu>