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/imap.rs

@@ -22,7 +22,6 @@ use crate::chat::{self, ChatId, ChatIdBlocked};
 use crate::config::Config;
 use crate::constants::{
     Blocked, Chattype, ShowEmails, DC_FETCH_EXISTING_MSGS_COUNT, DC_FOLDERS_CONFIGURED_VERSION,
-    DC_LP_AUTH_OAUTH2,
 };
 use crate::contact::{normalize_name, Contact, ContactId, Modifier, Origin};
 use crate::context::Context;
@@ -154,7 +153,6 @@ struct ImapConfig {
     pub lp: ServerLoginParam,
     pub socks5_config: Option<Socks5Config>,
     pub strict_tls: bool,
-    pub oauth2: bool,
     pub selected_folder: Option<String>,
     pub selected_mailbox: Option<Mailbox>,
     pub selected_folder_needs_expunge: bool,
@@ -243,7 +241,6 @@ impl Imap {
         lp: &ServerLoginParam,
         socks5_config: Option<Socks5Config>,
         addr: &str,
-        oauth2: bool,
         provider_strict_tls: bool,
         idle_interrupt: Receiver<InterruptInfo>,
     ) -> Result<Self> {
@@ -262,7 +259,6 @@ impl Imap {
             lp: lp.clone(),
             socks5_config,
             strict_tls,
-            oauth2,
             selected_folder: None,
             selected_mailbox: None,
             selected_folder_needs_expunge: false,
@@ -301,7 +297,6 @@ impl Imap {
             &param.imap,
             param.socks5_config.clone(),
             &param.addr,
-            param.server_flags & DC_LP_AUTH_OAUTH2 != 0,
             param
                 .provider
                 .map_or(param.socks5_config.is_some(), |provider| {
@@ -334,7 +329,7 @@ impl Imap {
 
         self.connectivity.set_connecting(context).await;
 
-        let oauth2 = self.config.oauth2;
+        let oauth2 = self.config.lp.oauth2;
 
         let connection_res: Result<Client> = if self.config.lp.security == Socket::Starttls
             || self.config.lp.security == Socket::Plain