kernel - Attempt to fix SMP races with ^T

* The ^T (ttyinfo()) code was still using critical-sections to protect
  its probe.  This no longer works.  Replace with appropriate hold's and
  tokens.

Reported-by: marino
kernel - Fix at least one 64-bit ddb backtrace issue

* Fix DDB support functions which were overflowing a 4-byte buffer when
  reading and writing 8-byte values.

Reported-by: tuxillo