Fix a longstanding bug in backtrace.cc: misuse of __cxa_demangle.

First, __cxa_demangle may realloc() provided buffer, so it must not be
allocated on stack. Second, user needs to use what __cxa_demangle returns,
and then free it.
Add CDU instrument.
Add noexcept to some SI:: methods.
Add Makefile symlink.
Comment about TextLayout::BackgroundMode.
Set default cursor to crosshair for all instruments.
Catch boost::io::bad_format_string in instruments/datatable.