take advantage of the rcorder -p argument to implement parallel
booting in rc.

According to the author non scientific tests:
on a Core 2 Duo with spinning disk:

| Services enabled | before | after | saving |
| 0                | 8s     | 8s    | 0      |
| 1                | 13s    | 13s   | 0      |
| 2                | 17s    | 13s   | 5      |
| 3                | 23s    | 13s   | 10     |
| 4                | 28s    | 13s   | 15     |
| 5                | 33s    | 13s   | 20     |

PR:		249192
MFC after:	3 weeks