MINOR: stream/cli: make "show sess" support filtering on front/back/server

MINOR: stream/cli: make "show sess" support filtering on front/back/server

With "show sess", particularly "show sess all", we're often missing the
ability to inspect only streams attached to a frontend, backend or server.
Let's just add these filters to the command. Only one at a time may be set.

One typical use case could be to dump streams attached to a server after
issuing "shutdown sessions server XXX" to figure why any wouldn't stop
for example.

(cherry picked from commit 5e558c172791978d41a2e3f07519bc49ee2337e3)
Signed-off-by: Willy Tarreau <w@1wt.eu>