Increase the default maximum stack walk

Increase the default maximum stack walk

lldb will only backtrace a fixed number of stack frames, as a
last-ditch attempt to avoid a runaway looping backtrace.  It's
unusual that anyone ends up depending on this final safety net in
years. I picked the original number of 300000 was picked by seeing
how many stack frames I could make in a small recursive function
on Darwin systems before using the default stack space.  Checking
again today on a modern system, I can exceed this limit & lldb will
not show the original invocation of the recursing call.  Double the
old value to cover this larger maximum possible stack frame count,
as a default value.
(`target.process.thread.max-backtrace-depth`)