dmesg: more colors, more fun

 * colorize subsystem prefix
 * colorize time
 * colorize by keywords (now "segfault" only)

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

 * enable --reltime
 * enable --color
 * a little bit cleanup main()

Signed-off-by: Karel Zak <kzak@redhat.com>
dmesg: use $PAGER for --human output, add --nopager

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