Before that, 'container-excursion' would call 'setns' even when the
target namespace is the one the caller is already in, which would fail.
* gnu/build/linux-container.scm (container-excursion): Introduce
'source' and 'target'. Compare the result of 'readlink' on these
instead of comparing file descriptors to decide whether to call
'setns'.
* tests/containers.scm ("container-excursion, same namespace"): New test.