[norec] Zero the stack in INIT_SHADOW_STACK() and explain why
[norec] Zero the stack in INIT_SHADOW_STACK() and explain why

During one of the recent commits, the code clearing the stack has been
dropped by mistake. Now a call to bzero() has been re-added in the right
place along with a comment explaining why it's mandatory.

vvaltchev/experimentOs,vvaltchev/experimentOs,vvaltchev/experimentOs,vvaltchev/experimentOs