A few small enhancements to the diagnose-unwind command.

Change the simple-minded stack walk to not depend on lldb to unwind
the first frame.

Collect a list of Modules and Addresses seen while backtracing (with
both methods), display the image list output for all of those modules,
plus disassemble and image show-unwind any additional frames that
the simple backtrace was able to unwind through instead of just the
lldb unwind algorithm frames.

Remove checks for older lldb's that didn't support -a for disassemble
or specifying the assembler syntax on x86 targets.