Write own init.go, provide examples

[cottand]

• write AWS' init.c but in Go
  • allows compatibility with kernels newer than 6.8 🎉
  • uses vendored sys.unix package which is where the large file count comes from
• add examples
• simplify API with a new buildEif function with sane defaults
• add an example for building an EIF with a non AWS kernel
  • currently broken, see Modern kernels aws/aws-nitro-enclaves-sdk-bootstrap#26 (comment)

note: in Go, prefer sys.unix over syscall