`runsc boot`: Do not re-exec when possible.

This drops caps using `syscall.AllThreadsSyscall6` instead of re-exec'ing.

A follow-up PR will do the same for the gofer process (which also re-execs
currently).

```
┌───────────────────────────┬─────────────────┬────────────────────────────┬──────────────────────────────────┐
│         Benchmark         │     before      │   no runsc boot re-exec    │ no runsc boot + no gofer re-exec │
├───────────────────────────┼─────────────────┼────────────────────────────┼──────────────────────────────────┤
│ docker run alpine true    │ 817ms           │ −8.4ms, ~ (p=0.131, n=100) │ −10.3ms, −1.26% (p=0.048, n=100) │
│ runsc create              │ 157ms           │ −4.2ms, ~ (p=0.123)        │ −9.0ms, −5.68% (p=0.019)         │
│ runsc create + start      │ 303ms           │ −10.2ms, ~ (p=0.081)       │ −19.9ms, −6.54% (p=0.002)        │
└───────────────────────────┴─────────────────┴────────────────────────────┴──────────────────────────────────┘
```