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>

src/jtag/jtag.h

@@ -737,4 +737,21 @@ int jtag_get_error(void);
  */
 int jtag_error_clear(void);
 
+/**
+ * Return true if it's safe for a background polling task to access the
+ * JTAG scan chain.  Polling may be explicitly disallowed, and is also
+ * unsafe while nTRST is active or the JTAG clock is gated off.,
+ */
+bool is_jtag_poll_safe(void);
+
+/**
+ * Return flag reporting whether JTAG polling is disallowed.
+ */
+bool jtag_poll_get_enabled(void);
+
+/**
+ * Assign flag reporting whether JTAG polling is disallowed.
+ */
+void jtag_poll_set_enabled(bool value);
+
 #endif /* JTAG_H */