Go back to storing die/attr strings in their trees

It's really not *that* much memory that we should bother constantly
refreshing the strings.  This way, the tooltip is free too.
Get rid of the conditionally-allocated strings

There were too many code hoops to let functions return either static
strings or allocated strings.  Just make them all allocated, using
g_strdup for the former statics.
Parse and display the .gnu_debugaltlink filename