[PATCH] debuginfod: add CORS support

[Frank Ch. Eigler]

Hi -

> This is my first attempt at implementing CORS support in debuginfod.

Looks good to me really; will wait for a glance from others.

> I should probably add or change tests. [...]

I wouldn't overthink it - just add a "-i" to any random curl command
and look for ACAO:*.

I reconfigured the debuginfod.elfutils.org server (at the reverse
proxy level) to emit that header already.  This may be enough for your
web gadget to start working by default.  I'm not sure the OPTIONS part
is needed for that.

- FChE