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/client/admin.go

@@ -139,6 +139,14 @@ func (a *API) Users(ctx context.Context) ([]User, error) {
 	return out.Users, err
 }
 
+// ChangePassword replaces the account password. The server requires the
+// current one, and the second factor when one is enrolled, then signs out
+// this account's other sessions.
+func (a *API) ChangePassword(ctx context.Context, current, next, totp string) error {
+	return a.do(ctx, http.MethodPost, "/api/v1/auth/password", map[string]string{
+		"current_password": current, "new_password": next, "totp": totp}, nil)
+}
+
 // EnrollTOTP starts second-factor enrolment and returns the shared secret
 // and its otpauth URI.
 func (a *API) EnrollTOTP(ctx context.Context) (secret, uri string, err error) {