osdmaptool: fix --pool option for --test-map-object mode

Commit 7f1b12f2ef97 ("osdmaptool: add --test-map-pgs mode") broke
--pool for --test-map-object mode.  Fix it, and improve --pool option
handling for both modes while at it (report strict_strtol() errors,
check if specified pool exists).

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
osdmaptool: add --pool option for --test-map-pgs mode to usage()

--test-map-pgs mode allows to map all pgs from either all pools or just
one pool.  Mention it in usage output.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
osdmaptool: add tests for --pool option

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>