Rename rumprun-xen to rumprun

To reflect the intent to support different stacks in future (eg. kvm),
rumprun-xen is renamed to just rumprun, with a mandatory STACK as its
first argument.

Signed-off-by: Martin Lucina <martin@lucina.net>
rumprun: Change delimiter for BLKSPEC and NETSPEC to comma

Will work much better for IPv6 and/or NFS.

Signed-off-by: Martin Lucina <martin@lucina.net>