logread: implement dumpfollow mode of operation

logread: implement dumpfollow mode of operation

This is basically a combination of the default (dump mode) and -f
(follow mode). Specifying -F makes logread first dump the log buffer and
then immediately start following it.

function                                             old     new   delta
packed_usage                                       30412   30443     +31
logread_main                                         491     497      +6

Signed-off-by: Phil Sutter <phil.sutter@viprinet.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>