CLEANUP: cli: make "show fd" use a const connection to access other fields

CLEANUP: cli: make "show fd" use a const connection to access other fields

Over time the code has uglified, casting fdt.owner as a struct connection
for about everything. Let's have a const struct connection* there and take
this opportunity for passing all fields as const as well.

Additionally a misplaced closing parenthesis on the output was fixed.

(cherry picked from commit eb0595d039b5e5c56bf3f574ec7e364d926c406b)
[wt: s/sock_conn_iocb/conn_fd_handler]
Signed-off-by: Willy Tarreau <w@1wt.eu>