rc: implement parallel boot
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