Return better "error" values for UWX_BOTTOM and UWX_ABI_FRAME in
unw_step(). Both errors denote the end of a stack trace (i.e. no
prior frame), but are otherwise not error conditions.
Have db_trace() return 0 when the trace ends due to one of these
return codes as they are really normal termination conditions.

This change especially improves the output of the "show thread"
command in DDB when there are threads in fork_trampoline() and
previously db_trace() would return an error, causing the show
command to emit '***'.