Allow hostname used by XRootD to be overridden by environment variable

[bbockelm]

In Kubernetes, the hostname the service is accessible by from the outside world is often quite decoupled from the UTS name or the reverse DNS lookup.

XRootD lacks the ability to completely override the hostname environment variable, including in the advertisements to the rest of the cluster. This patch series introduces the XRDHOST env var enabling the override.

Upstream PR: xrootd#1868

[bbockelm]

Note: upstream rejected this idea in 2022 and then re-implemented in 2024 using a different environment variable name (XRDNET_IDENTITY). See 1e59e5c.

It's unclear if the upstream re-implementation is as complete -- we need to re-test, especially with the cmsd (I don't see anything in the upstream commit that would fix the cmsd issues where it advertises the desired hostname instead of having the remote manager do a reverse-DNS lookup by default).

Multiple OSG sites now depend on the existing environment variable for functionality, as well as within Pelican. If the cmsd is also working upstream, we'll want to provide a transition period where both env vars are present.

[matyasselmeci]

How should we go about testing XRDNET_IDENTITY? Looks like it has been in XRootD since 5.7.1, and Pelican already requires 5.8.2. Should we make a ticket in PelicanPlatform/pelican to do a search & replace of XRDHOST to XRDNET_IDENTITY, or is that premature?