the previous frame is in the usual place even for traps, interrupts
and syscalls in the kernel, because the assembly language stubs
don't change the frame pointer. The previous frame is just not for
the calling function. We may as well depend on this as on magic to
determine the trap frame address. The magic is in FRAME_SAVED_PC()
which elides the correct number of stubs (1) to go back to a pc that
matches the previous frame.
Removing fbsd_kern_frame_chain() fixes bugs in it. Xsyscall was
misspelled as _Xsyscall (gdb removes one leading underscore), so
the tf_syscall frame type was never found. This was harmless
because tf_normal works in all cases in fbsd_kern_frame_chain()
and Xsyscall is spelled correctly in fbsd_kern_frame_saved_pc()
where it matters. There were style bugs on almost every line,
starting with a primary indent of 7.
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