mac-sys-utils:  Fix 68K foreign stack space calc

Check for zero from ThreadCurrentStackSpace() (which can happen when the
stack wasn't created by the Thread Manager), and don't add the 68K fudge
factor, since the caller is checking for zero as a special case, not 10.
Genie:  Fix comments in dispatch_68k_system_call()

The stack frame begins with the backlink followed by the return address.
Genie:  Write the correct 68K stack footer slots

Write the frame pointer and return address into the specified slots, not
over the stack-limit and previous-stack members.
Merge branch '68k-syscall-stack-fixes'
Genie:  Fix whitespace in system_call.68k.cc

Also delete redundant comment-introducing semicolons.
Genie:  Fix whitespace in system_call.ppc.cc
Genie, relix:  Don't allocate pid 1 syscall stack
relix:  Null-check the syscall stack memory store

Allocation of a system call stack is now optional.
Genie:  Null-check syscall stack in 68K dispatcher
Merge branch 'optional-syscall-stack'
Genie:  Limit syscall stack use to 68K on Mac OS 9
relix:  Enable system call stacks