all: make vendored copy of reexec (#28382)

a little copying is better than a little dependency

-- go proverb

We have this dependency on docker, a.k.a moby: a gigantic library, and we only need ~70 LOC,
so here I tried moving it inline instead.

Co-authored-by: Felix Lange <fjl@twurst.com>

build/update-license.go

@@ -65,10 +65,8 @@ var (
 		"vendor/", "tests/testdata/", "build/",
 
 		// don't relicense vendored sources
-		"cmd/internal/browser",
 		"common/bitutil/bitutil",
 		"common/prque/",
-		"consensus/ethash/xor.go",
 		"crypto/blake2b/",
 		"crypto/bn256/",
 		"crypto/bls12381/",
@@ -78,6 +76,7 @@ var (
 		"log/",
 		"metrics/",
 		"signer/rules/deps",
+		"internal/reexec",
 
 		// skip special licenses
 		"crypto/secp256k1", // Relicensed to BSD-3 via https://github.com/ethereum/go-ethereum/pull/17225