patch rfc: debuginfod -Z (generalized archive) support

[Frank Ch. Eigler]

Hi -

> Yes, lets make =CMD optional. But why assume "cat"? Can't you then
> simply use fopen instaed of popen in the code to get the FILE* to pass
> to archive_read_open_FILE? Or even just open and archive_read_open_fd?

One reason: the defer_dtor<>-based auto-closing of these objects is
different.  (popens must be closed with pclose not something else.)  A
robust auto-closer is needed because of all the exceptions we may
trigger.

- FChE