Improve swap handling

Modify the swap script so it starts before and stops after localmount.
Also, on linux, this script should skip mounting swap on loopback.

Add a swapfiles script which runs after localmount and is designed to
activate additional swap space which could not be activated before
localmount was run. This includes loopback swap on linux and swapfiles
which are on local file systems that were not mounted when swap was
activated initially.

The start code does reactivate swap that is already active, but this is
not an error condition.

Reported-by: Giampaolo Tomassoni <giampaolo@tomassoni.biz>
X-Gentoo-Bug: 401003
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=401003

runlevels/Makefile

@@ -1,5 +1,5 @@
 BOOT=		bootmisc fsck hostname localmount \
-		root swap sysctl urandom ${BOOT-${OS}}
+		root swap swapfiles sysctl urandom ${BOOT-${OS}}
 DEFAULT=	local netmount
 SHUTDOWN=	savecache ${SHUTDOWN-${OS}}
 SYSINIT=	${SYSINIT-${OS}}