Hello,

This patch some what brutally splits the single mips_mdebug unwinder 
three ways into:

mdebug: only used when post prologue and mdebug
mips16: used when mips16 (including the prologue)
mips32: used when mips32 (including the prologue)

and also adds:

stub: for trampolines and the like

While I've completed the task of getting each sniffer function to just 
detect its own frame, the task of gutting(1) the mdebug, mips16 and 
mips32 unwind_cache functions is still sitting on the table.  Anyone up 
for a challenge?

Andrew

(1) There's no better word: inline function call; eliminate dead code; 
repeat.