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 signing and
loading signed manifests 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 OpenPGP secret key; a protected key's passphrase
comes from MFER_SIGN_KEY_PASSPHRASE or a prompt on the terminal, and is
checked before any file is read. Verification keeps the rules of the
--require-signature fix: one primary key in the embedded block, counted
from its packets so that keys the library skips count too, exactly one
signature, made by that key or one of its subkeys, and signer equal to
its fingerprint. An armored key or signature must be one block and
nothing else.

Model: opus-5-5

README.md

@@ -257,8 +257,9 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
 
 - Should the manifest signature format be GnuPG signatures, or those from
   OpenBSD's signify (of which there is a good
-  [golang implementation](https://github.com/frankbraun/gosignify))? Still open,
-  as question 10 on [issue 82](https://git.eeqj.de/sneak/mfer/issues/82).
+  [golang implementation](https://github.com/frankbraun/gosignify))? Settled
+  under question 10 on [issue 82](https://git.eeqj.de/sneak/mfer/issues/82):
+  OpenPGP signatures, which mfer makes and checks itself without running `gpg`.
 
 - Should the on-disk serialization format be proto3 or json? Settled: it is
   proto3, see `docs/FORMAT.md` and `mfer/mf.proto`.
@@ -296,6 +297,14 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
     - leaves out hidden files unless given `--include-dotfiles`, and symlinks
       unless given `--follow-symlinks`, which lists each symlink to a file under
       its own name with the contents of the file it points to
+- `mfer gen --sign-key key.asc` / `mfer freshen --sign-key key.asc`
+    - signs the manifest with the OpenPGP secret key in `key.asc`, armored or
+      binary, as `gpg --export-secret-keys` writes it; `MFER_SIGN_KEY` names the
+      file too. mfer signs it itself and does not need `gpg`. A file holding
+      more than one key is refused, and a key held only on a smartcard cannot
+      sign
+    - takes a protected key's passphrase from `MFER_SIGN_KEY_PASSPHRASE`, or
+      else asks for it at the terminal
 - `mfer fetch https://example.com/stuff/`
     - fetches `/stuff/index.mf` and downloads all files listed in manifest into
       the current directory, or the one given with `--dest`, and assures