prevent abort via polling during jtag_reset

Observed:

  openocd: core.c:318: jtag_checks: Assertion `jtag_trst == 0' failed.

The issue was that nothing disabled background polling during calls
from the TCL shell to "jtag_reset 1 1".  Fix by moving the existing
poll-disable mechanism to the JTAG layer where it belongs, and then
augmenting it to always pay attention to TRST and SRST.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>