dmesg: add --decode to print readable facility and level

dmesg: add --decode to print readable facility and level

 # dmesg --decode
 ...
 kern  :info  : [53335.743185] PM: resume of devices complete after 2496.795 msecs
 kern  :debug : [53335.743593] PM: Finishing wakeup.
 kern  :warn  : [53335.743595] Restarting tasks ... done.
 kern  :info  : [53335.790452] video LNXVIDEO:00: Restoring backlight state
 kern  :debug : [53336.418576] e1000e 0000:00:19.0: irq 45 for MSI/MSI-X

Signed-off-by: Karel Zak <kzak@redhat.com>