Before:
```
Benchmark #1: ./skipcpio/skipcpio test-5.10.15-200.fc33.x86_64.img >/dev/null
Time (mean ± σ): 125.5 ms ± 0.9 ms [User: 97.4 ms, System: 27.9 ms]
Range (min … max): 124.8 ms … 129.4 ms 23 runs
```
After:
```
Benchmark #1: ./skipcpio/skipcpio test-5.10.15-200.fc33.x86_64.img >/dev/null
Time (mean ± σ): 12.2 ms ± 0.3 ms [User: 2.7 ms, System: 9.5 ms]
Range (min … max): 11.7 ms … 13.6 ms 212 runs
```
Besides the speedup, skipcpio now parses the cpio header and is not
falsely ending when the early cpio payload contains `TRAILER!!!`.
Fixes: https://github.com/dracutdevs/dracut/issues/1123
Harald Hoyer