dmesg: add --reltime to print human readable deltas

dmesg: add --reltime to print human readable deltas

[Jul20 09:50] device vnet0 entered promiscuous mode
[  +0.002248] virbr0: topology change detected, propagating
[  +0.000017] virbr0: port 2(vnet0) entered forwarding state
[  +0.000036] virbr0: port 2(vnet0) entered forwarding state
[  +0.001663] ADDRCONF(NETDEV_CHANGE): virbr0: link becomes ready
[ +10.480914] vnet0: no IPv6 routers present
[Jul20 14:35] xhci_hcd 0000:0e:00.0: WARN Event TRB for slot 3 ep 5 with no TDs queued?
[Jul20 19:23] EXT4-fs (sda2): re-mounted. Opts: (null)
[  +6.052980] EXT4-fs (sda2): re-mounted. Opts: (null)

Suggested by Linus:
http://thread.gmane.org/gmane.linux.kernel/1290886/focus=1296138

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