Merge robustness improvements for the ALTERA JTAG UART driver from

Merge robustness improvements for the ALTERA JTAG UART driver from
CheriBSD, which attempt to work around an inherent race in the UART's
control-register design in detecting whether JTAG is currently,
present, which will otherwise lead to moderately frequent output
drops when running in polled rather than interrupt-driven operation.
Now, these drops are quite infrequent.

commit 9f33fddac9215e32781a4f016ba17eab804fb6d4
Author: Robert N. M. Watson <robert.watson@cl.cam.ac.uk>
Date:   Thu Jul 16 17:34:12 2015 +0000

    Add a new sysctl, hw.altera_jtag_uart.ac_poll_delay, which allows the
    (default 10ms) delay associated with a full JTAG UART buffer combined
    with a lack of a JTAG-present flag to be tuned.  Setting this higher
    may cause some JTAG configurations to be more reliable when printing
    out low-level console output at a speed greater than the JTAG UART is
    willing to carry data.  Or it may not.

commit 73992ef7607738b2973736e409ccd644b30eadba
Author: Robert N. M. Watson <robert.watson@cl.cam.ac.uk>
Date:   Sun Jan 1 15:13:07 2017 +0000

    Minor improvements to the Altera JTAG UART device driver:

    - Minor rework to the logic to detect JTAG presence in order to be a bit
      more resilient to inevitable races: increase the retry period from two
      seconds to four seconds for trying to find JTAG, and more agressively
      clear the miss counter if JTAG has been reconnected.  Once JTAG has
      vanished, stop prodding the miss counter.

    - Do a bit of reworking of the output code to frob the control register
      less by checking whether write interrupts are enabled/disabled before
      changing their state.  This should reduce the opportunity for races
      with JTAG discovery (which are inherent to the Altera
      hardware-software interface, but can at least be minimised).

    - Add statistics relating to interrupt enable/disable/JTAG
      discovery/etc.

    With these changes, polled-mode JTAG UART ttys appear substantially
    more robust.

MFC after:	1 week
Sponsored by:	DARPA, AFRL