rec: Delay the creation of RPZ threads until we have dropped privileges

rec: Delay the creation of RPZ threads until we have dropped privileges

On Linux/glibc, calling `set*id()` from a thread results in the other
threads being sent the `SIGRT_1` signal so they are aware that they
should switch credentials too, because `POSIX` requires that all threads
use the same credentials but Linux actually handles it per thread.
The reception of the signal interrupts the current `syscall` with
`EINTR`, causing the loading of the `RPZ` zone to fail.