[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.

ntoskrnl/kd64/kdinit.c

@@ -26,8 +26,8 @@
  *
  * See also: kd\kdio.c
  */
-static SIZE_T
-INIT_FUNCTION
+static INIT_FUNCTION
+SIZE_T
 KdpGetMemorySizeInMBs(IN PLOADER_PARAMETER_BLOCK LoaderBlock)
 {
     PLIST_ENTRY ListEntry;
@@ -67,8 +67,8 @@ KdpGetMemorySizeInMBs(IN PLOADER_PARAMETER_BLOCK LoaderBlock)
 }
 
 /* See also: kd\kdio.c */
-static VOID
-INIT_FUNCTION
+static INIT_FUNCTION
+VOID
 KdpPrintBanner(IN SIZE_T MemSizeMBs)
 {
     DPRINT1("-----------------------------------------------------\n");
@@ -340,7 +340,7 @@ KdInitSystem(IN ULONG BootPhase,
     }
     else
     {
-        /* Called from a bugcheck or a re-enable. Save the Kernel Base */
+        /* Called from a bugcheck or a re-enable. Save the Kernel Base. */
         KdVersionBlock.KernBase = (ULONG64)(LONG_PTR)PsNtosImageBase;
 
         /* Unconditionally enable KD */