rc.d/svscan: Use -f to daemon(8) again.

• This reverts the change from r247267 which removed it.
• The upstream svscanboot script closes stdout, stdin and stderr. Our script should do the same. Doing this does not lose any output since the output is all piped to the proper logger. This fixes hangs when combined with Salt [1].

Reported by: C. R. Oldham <cro@ncbt.org> [1]