dmesg: add --time-format option

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
dmesg: add iso-8601 time format

The ISO-8601 should be welcomed by anyone who tries to parse dmesg time
stamp, and compare them across servers.  Time format tries to imitate
coreutils 'date --iso-8601=ns' output, but instead of having nanoseconds
the dmesg is using microseconds.

Reference: http://www.cs.tut.fi/~jkorpela/iso8601.html
Addresses: http://xkcd.com/1179/
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
docs: add --time-format option and ISO-8601 format to manual

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
dmesg: make usage() a little bit shorter

The dmesg has rather long usage, so drop the timestamp format options
which are aliases of the more generic options.  Old options will remain
working as earlier.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>