Sign and verify manifests in Go with OpenPGP instead of running gpg (closes #181)

mfer ran the gpg binary to sign, export keys and verify, so it failed
wherever gpg is missing. It now uses github.com/ProtonMail/go-crypto/openpgp.
--sign-key and MFER_SIGN_KEY name a file holding one version 4 OpenPGP
secret key; a protected key's passphrase comes from
MFER_SIGN_KEY_PASSPHRASE or a terminal prompt. gen and freshen check that
the key can sign before they read any file. Verification keeps the rules
of the --require-signature fix: one primary key in the embedded block,
counted from its packets, exactly one signature, made by that key or a
subkey, and signer equal to its fingerprint. The embedded block may hold
no DSA key and no secret key, and an armored field must be one
well-formed block.

Model: opus-5-5

mfer/deserialize_fuzz_test.go

@@ -19,14 +19,6 @@ import (
 // input and of the decompressed data it may read, plus room for the
 // decoder's window buffers. A panic or a hang fails the test on its own.
 func FuzzNewManifestFromReader(f *testing.F) {
-	// A signed manifest makes the parser write the key and signature to a
-	// temporary directory and run gpg on them. With gpg off the PATH and
-	// temporary files kept in the test's own directory, no process is
-	// started and nothing is written elsewhere; such input ends in an
-	// error instead.
-	f.Setenv("PATH", "")
-	f.Setenv("TMPDIR", f.TempDir())
-
 	f.Fuzz(func(t *testing.T, data []byte) {
 		var before, after runtime.MemStats