Fix single sign-on from the web, and make passwords changeable

Why: two questions about operating this turned up two gaps. "Where do I get
the admin password" had no answer once the line scrolled out of the log,
and "how do I set up OIDC" ran straight into a sign-in link that could not
work.

What:
- /oidc/login with no parameters now signs a browser in and ends in a
  session cookie. It previously required the user code of a waiting client,
  so the link on the web sign-in page always failed. With a code it still
  approves a device, and the two paths no longer overlap: an empty user
  code matches no pending device.
- POST /api/v1/auth/password, `keeporg passwd`, and a form on /web/vaults.
  Changing a password requires the current one, and the one-time code when
  a second factor is enrolled, and revokes that account's other sessions.
  Until now nothing could change a password, which made the advice printed
  at first start impossible to follow.
- keeporgd -reset-admin, for when that first password was not kept: a new
  one printed once, any second factor cleared, sessions revoked. It writes
  the state document directly and says to stop the server first.
- The client read stdin through a new buffered reader per prompt, so the
  first prompt swallowed the rest and anything asking twice could not be
  scripted. One shared reader now. A prompt for a password being set no
  longer reads $KEEPORG_PASSWORD either; that variable holds the password
  you sign in with, and taking it here would have set the new password to
  the old one.
- HANDOVER gains a first-sign-in section and an OIDC setup section covering
  the redirect URI, the config block, both entry points and how an identity
  becomes an account.

Proved: OIDC now has tests, against a stub provider that serves discovery,
authorization, token and userinfo. They cover the web sign-in that was
broken, device approval, linking an existing account by username, the
refusal when auto_create is off, and a forged callback state. Eight more
cover the password change. Checked by hand with the real binaries: the
first-start password logs once and works, -reset-admin replaces it while
the old one stops working, and `keeporg passwd` changes it from a pipe.
Server 0.5.0, desktop clients 0.3.0.

internal/store/model.go

@@ -154,8 +154,14 @@ type Session struct {
 // Android: the client shows a short user code, the browser approves it, the
 // client polls until a session token appears.
 type DeviceAuth struct {
-	DeviceCode   string    `json:"device_code"`
-	UserCode     string    `json:"user_code"`
+	DeviceCode string `json:"device_code"`
+	// UserCode is the short code a person reads out to approve a client.
+	// A browser signing in to the web interface has none: there is no
+	// second device to approve.
+	UserCode string `json:"user_code"`
+	// Web marks a sign-in that ends in a cookie rather than in a token a
+	// waiting client polls for.
+	Web          bool      `json:"web,omitempty"`
 	Label        string    `json:"label,omitempty"`
 	SessionToken string    `json:"session_token,omitempty"`
 	OIDCState    string    `json:"oidc_state,omitempty"`
@@ -227,8 +233,13 @@ func (s *State) UserByOIDC(issuer, subject string) *User {
 	return nil
 }
 
-// DeviceByUserCode finds a pending device login by the code shown to the user.
+// DeviceByUserCode finds a pending device login by the code shown to the
+// user. An empty code matches nothing, so a web sign-in, which has no user
+// code, is never mistaken for a device waiting to be approved.
 func (s *State) DeviceByUserCode(code string) *DeviceAuth {
+	if code == "" {
+		return nil
+	}
 	for _, d := range s.Devices {
 		if d.UserCode == code {
 			return d