hurd default pager/swap space changes

[Roland McGrath <roland@frob.com>]

I've made some minor changes to the hurd having to do with swap space.
(These changes are in cvs, but not yet propagated to the anoncvs mirror.)

The `$(default-pager)' magic token /boot/servers.boot is now obsolete.  It
has not in fact done anything for quite some time, and not it is gone.
Having the token in your file with the new hurd will be harmless, and not
having it there with the current hurd is also harmless.

I made the default pager notice when you try to add the same swap partition
twice, and refuse the second time.

I made the -a option to swapon/swapoff work.  Now you can just lines in
/etc/fstab with type "swap" for swap partition, as on Linux.  rc now runs
`swapon -a' first thing at boot time.

I think we should stop telling people to use $(add-paging-partition) et al
in the boot script (/boot/servers.boot) and instead to boot up with no swap
and then use swapon.  This is what Linux and other systems usually do nowadays.

Note that for a native install, you'll need to run MAKEDEV for the swap
partition devices before you can run swapon.  But I expect everyone has at
least enough memory to do that.

Note that using swapon is like $(add-paging-file) rather than
$(add-linux-paging-file) or $(add-raw-paging-file).  That is, it recognizes
a Linux swap signature page but will use the partition even if it doesn't
have one.  If people care about the extra safeguard, probably the thing to
do is have swapon (controlled by a switch) read the first page of the
device and check it before calling the default pager at all.