[FREELDR]: Jump to a standard 1-parameter STDCALL kernel entrypoint instead of a...

[FREELDR]: Jump to a standard 1-parameter STDCALL kernel entrypoint instead of a FASTCALL double-parameter entrypoint.
[NTOS]: Make KiSystemStartup the real C entrypoint of the kernel, and move the "Am I being booted by FreeLDR" logic inside it -- it will then call KiRosPrepareForSystemStartup as earlier.
[NTOS]: Move the Double Fault and Boot Stack declaration in C code, with the proper alignment attribute.
[NTOS]: Although the concern that KiSystemStartup cannot be 100% C since it modifies ESP is real (Thomas' original fix of Alex's code), we don't need that much of it in assembly. Instead, write a simple trampoline (KiSwitchToBootStack) inline which switches stacks and jumps to a second-stage C function.
[NTOS]: Completely remove boot.S as it isn't needed anymore, ReactOS startup is back to being (nearly) 100% C.

svn path=/trunk/; revision=45152