dmesg: support for additional human readable timestamp

dmesg: support for additional human readable timestamp

The dmesg logs have timestamps in seconds since boot format which can
be converted to other formats. However, there is no option to include
both the original timestamp as present in the buffer along with the
converted timestamp in the specified format.

This change updates the --time-format option in dmesg to enable the
user to specify it multiple times with different formats with each
input providing a timestamp format among the currently supported
choices. This enables seeing the dmesg logs in both seconds since boot
and human readable format simultaneously in each row of the log output.
The sequence of timestamp format follows the order
in which the user provides the desired formats.

Signed-off-by: Rishabh Thukral <rthukral@arista.com>