Validate and simplify LoginParam struct

This makes sure that under normal circumstances the LoginParam struct
is always fully validated, ensure future use does not have to be
careful with this.

The brittle handling of `server_flags` is also abstraced away from
users of it and is now handled entirely internally, as the flags is
really only a boolean a lot of the flag parsing complexity is removed.
The OAuth2 flag is moved into the ServerLoginParam struct as it really
belongs in there.

src/smtp.rs

@@ -11,7 +11,6 @@ use async_smtp::{smtp, EmailAddress, ServerAddress};
 use async_std::task;
 
 use crate::config::Config;
-use crate::constants::DC_LP_AUTH_OAUTH2;
 use crate::contact::{Contact, ContactId};
 use crate::events::EventType;
 use crate::login_param::{
@@ -103,7 +102,6 @@ impl Smtp {
             &lp.smtp,
             &lp.socks5_config,
             &lp.addr,
-            lp.server_flags & DC_LP_AUTH_OAUTH2 != 0,
             lp.provider
                 .map_or(lp.socks5_config.is_some(), |provider| provider.strict_tls),
         )
@@ -117,7 +115,6 @@ impl Smtp {
         lp: &ServerLoginParam,
         socks5_config: &Option<Socks5Config>,
         addr: &str,
-        oauth2: bool,
         provider_strict_tls: bool,
     ) -> Result<()> {
         if self.is_connected().await {
@@ -146,7 +143,7 @@ impl Smtp {
         let tls_config = dc_build_tls(strict_tls);
         let tls_parameters = ClientTlsParameters::new(domain.to_string(), tls_config);
 
-        let (creds, mechanism) = if oauth2 {
+        let (creds, mechanism) = if lp.oauth2 {
             // oauth2
             let send_pw = &lp.password;
             let access_token = dc_get_oauth2_access_token(context, addr, send_pw, false).await?;