[NTOS:KD/KDBG] Code enhancements for KDBG/KD.

[NTOS:KD/KDBG] Code enhancements for KDBG/KD.

- Introduce KdpScreenAcquire() / KdpScreenRelease() helpers that allow
  to correctly acquire or release the screen when using INBV for
  "Screen-Mode" debugging.

- Introduce KdpAcquireLock() / KdpReleaseLock() helpers so as to reduce
  the copy-pasted code required when acquiring spin-locks in the KD debug
  logger functions.

- Close the opened KdpLogFileHandle file in the KdpInitDebugLog() logger
  in case we failed to create the logger writer thread.
  Also use explicit kernel-mode handle when opening the log file.

- static-ify some local variables, remove few hardcoded values, and
  minor formatting.

- Correctly define the INIT_FUNCTION's.