tmpfiles: if we get ENOENT when opening /proc/self/fd/, check if /proc is mounted

let's return ENOSYS in that case, to make things a bit less confusng.

Previously we'd just propagate ENOENT, which people might mistake as
applying to the object being modified rather than /proc/ just not being
there.

Let's return ENOSYS instead, i.e. an error clearly indicating that some
kernel API is not available. This hopefully should put people on a
better track.

Note that we only do the procfs check in the error path, which hopefully
means it's the less likely path.

We probably can add similar bits to more suitable codepaths dealing with
/proc/self/fd, but for now, let's pick to the ones noticed in #14745.

Fixes: #14745