- unify UNW_IA64_AR_BSP and UNW_IA64_BSP

- debuggers need additional frame info:
	- ability to determine first non-prologue instruction
	  (gdb calls this skip_prologue) => this doesn't really
	  make much sense for optimized code: there may be now
	  single IP at which the .body starts; the gdb maintainers
	  are aware of this and are planning to modify gdb such that
	  skip_prologue can always return 0
	- ability to find out whether and where a frame saved a register

- finish & test unwinding across signal frame (pay attention that signal
  frame gets updated)