target: improve robustness of polling and reexamination

target: improve robustness of polling and reexamination

When a target was present on OpenOCD start but later disappeared for
whatever reason (typically unstable connection or target going to
sleep) and reappeared only for a brief period of time, reexamination
would fail, and poll would no longer run. This patch fixes it.

Change-Id: I61f9b5a3f366a761320e233f4e1689f926b5556d
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2370
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins