debuginfod: PR27917 - protect against federation loops

debuginfod: PR27917 - protect against federation loops

If someone misconfigures a debuginfod federation to have loops, and
a nonexistent buildid lookup is attempted, bad things will happen,
as is documented.

This patch aims to reduce the risk by adding an option to debuginfod
that functions kind of like an IP packet's TTL: a limit on the length of
XFF: header that debuginfod is willing to process. If X-Forwarded-For:
exceeds N hops, it will not delegate a local lookup miss to upstream
debuginfods.

Commit ab38d167c40c99 causes federation loops for non-existent resources
to result in multiple temporary deadlocks, each lasting for
$DEBUGINFOD_TIMEOUT seconds. Since concurrent requests for each unique
resource are now serialized, federation loops can result in one server
thread waiting to acquire a lock while the server thread holding the
lock waits for the first thread to respond to an http request.

This PR can help protect against the above multiple temporary deadlocks
behaviour. Ex. if --forwarded-ttl-limit=0 then the timeout behaviour of
local loops should be avoided.

https://sourceware.org/bugzilla/show_bug.cgi?id=27917

Signed-off-by: Di Chen <dichen@redhat.com>