fsck: remove fsckd again, but keep the door open for external replacement

fsck: remove fsckd again, but keep the door open for external replacement

For a longer discussion see this:

http://lists.freedesktop.org/archives/systemd-devel/2015-April/030175.html

This introduces /run/systemd/fsck.progress as a simply
AF_UNIX/SOCK_STREAM socket. If it exists and is connectable we'll
connect fsck's -c switch with it. If external programs want to get
progress data they should hence listen on this socket and will get
all they need via that socket. To get information about the connecting
fsck client they should use SO_PEERCRED.

Unless /run/systemd/fsck.progress is around and connectable this change
reverts back to v219 behaviour where we'd forward fsck output to
/dev/console on our own.