dmesg: make time format parsing to use enum bit field

This will pave way to add single option to define all time formats, such
as --time-format=[ctime|reltime|delta|none].

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