mon/OSDMonitor: restructure command flow

- boilerplate at the top
- reply and update labels
- default r == 0
- explicitly fall out invalid or unrecognized commands in final else block

Signed-off-by: Sage Weil <sage@inktank.com>
mon/PGMonitor: reflow command methods

- simplify dump_stuck_pg_stats() method
- default r = 0
- explicit update, reply labels
- avoid deep nesting when many error checks
- clean up return codes

Signed-off-by: Sage Weil <sage@inktank.com>