pool: add Io mode into mover's status line

we got a request from our admins to add READ/WRITE info
to the output of 'mover ls'.

Acked-by: Gerd Behrmann
Target: master, 2.10, 2.11
Require-book: no
Require-notes: yes
(cherry picked from commit 7f61122516e42f3901dd03f6676057599869c1b0)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Merge pull request #1156 from kofemann/mover-io-mode-2.10

pool: add Io mode into mover's status line