Write encrypt and decrypt output only once they succeed (closes #143)

`secret encrypt` and `secret decrypt` emptied the file named by
`--output` before reading any input, so a failure left it empty, and a
file given as both `--input` and `--output` was emptied before it was
read. They now write into a temporary file of mode 0600 next to it, or
next to the file it is a symbolic link to, and rename that over it only
on success, with the new `secret.WriteFileAtomicFunc`. An output file
that cannot be written still fails. Ctrl-C, SIGTERM or a hangup removes
the temporary file. A device or pipe, such as `/dev/null`, is written to
directly.

Judgement calls: the device and pipe case; a command started with
`nohup` still ignores a hangup.

Model: opus-5-5

internal/cli/leftovers_test.go

@@ -52,10 +52,10 @@ func TestLeftoversRemovedByNextChangingCommand(t *testing.T) {
 	}
 
 	for _, file := range []string{
-		testStateDir + "/.currentvault.tmp-4",
-		vaultDir + "/.current-unlocker.tmp-5",
-		secretDir + "/.current.tmp-6",
-		secretDir + "/versions/" + versions[0] + "/.metadata.age.tmp-7",
+		testStateDir + "/.tmp-4",
+		vaultDir + "/.tmp-5",
+		secretDir + "/.tmp-6",
+		secretDir + "/versions/" + versions[0] + "/.tmp-7",
 	} {
 		require.NoError(t, afero.WriteFile(fs, file,
 			[]byte("partial"), secret.FilePerms))