Do no isolate in case of emergency or severe problems

Do no isolate in case of emergency or severe problems

This patch changes local-fs.target and systemd-fsck to not use
"isolate" when going into emergency.
This fixes https://bugzilla.redhat.com/show_bug.cgi?id=810722

The motivation is, that when something wents wrong, we should
keep everything as it is, to let the user fix the problem. When
isolating we stop a lot of services and therefore change the
system heavily so that it gets harder for the user to fix.

An example is a crypted partition. When the fsck in a crypted
partition fails, it previously used "emergency/start/isolate"
which stops cryptsetup. Therefore if the user tries to fsck
e.g. /dev/mapper/luks-356c20ae-c7a2-4f1c-ae1d-1d290a91b691
as printed by the failing fsck, then it will not find this
device (because it got closed).

So please apply this patch to let the user see the failing
situation.

Thanks!

[zj: removed dead isolate param from start_target().]

https://bugs.freedesktop.org/show_bug.cgi?id=49463
https://bugzilla.redhat.com/show_bug.cgi?id=810722