configure: try multiple servers for each protocol

LoginParamNew structure, which contained possible IMAP and SMTP
configurations to try is replaced with uniform vectors of ServerParams
structures. These vectors are initialized from provider database, online
Mozilla or Outlook XML configuration or user entered parameters.

During configuration, vectors of ServerParams are expanded to replace
unknown values with all possible variants, which are tried one by one
until configuration succeeds or all variants for a particular protocol
(IMAP or SMTP) are exhausted.

ServerParams structure is moved into configure submodule, and all
dependencies on it outside of this submodule are removed.

src/configure/server_params.rs

@@ -0,0 +1,115 @@
+//! Variable server parameters lists
+
+use crate::provider::{Protocol, Socket};
+
+/// Set of variable parameters to try during configuration.
+///
+/// Can be loaded from offline provider database, online configuraiton
+/// or derived from user entered parameters.
+#[derive(Debug, Clone)]
+pub(crate) struct ServerParams {
+    /// Protocol, such as IMAP or SMTP.
+    pub protocol: Protocol,
+
+    /// Server hostname, empty if unknown.
+    pub hostname: String,
+
+    /// Server port, zero if unknown.
+    pub port: u16,
+
+    /// Socket security, such as TLS or STARTTLS, Socket::Automatic if unknown.
+    pub socket: Socket,
+
+    /// Username, empty if unknown.
+    pub username: String,
+}
+
+impl ServerParams {
+    pub(crate) fn expand_usernames(mut self, addr: &str) -> Vec<ServerParams> {
+        let mut res = Vec::new();
+
+        if self.username.is_empty() {
+            self.username = addr.to_string();
+            res.push(self.clone());
+
+            if let Some(at) = addr.find('@') {
+                self.username = addr.split_at(at).0.to_string();
+                res.push(self);
+            }
+        } else {
+            res.push(self)
+        }
+        res
+    }
+
+    pub(crate) fn expand_hostnames(mut self, param_domain: &str) -> Vec<ServerParams> {
+        let mut res = Vec::new();
+        if self.hostname.is_empty() {
+            self.hostname = param_domain.to_string();
+            res.push(self.clone());
+
+            self.hostname = match self.protocol {
+                Protocol::IMAP => "imap.".to_string() + param_domain,
+                Protocol::SMTP => "smtp.".to_string() + param_domain,
+            };
+            res.push(self.clone());
+
+            self.hostname = "mail.".to_string() + param_domain;
+            res.push(self);
+        } else {
+            res.push(self);
+        }
+        res
+    }
+
+    pub(crate) fn expand_ports(mut self) -> Vec<ServerParams> {
+        // Try to infer port from socket security.
+        if self.port == 0 {
+            self.port = match self.socket {
+                Socket::SSL => match self.protocol {
+                    Protocol::IMAP => 993,
+                    Protocol::SMTP => 465,
+                },
+                Socket::STARTTLS | Socket::Plain => match self.protocol {
+                    Protocol::IMAP => 143,
+                    Protocol::SMTP => 587,
+                },
+                Socket::Automatic => 0,
+            }
+        }
+
+        let mut res = Vec::new();
+        if self.port == 0 {
+            // Neither port nor security is set.
+            //
+            // Try common secure combinations.
+
+            // Try STARTTLS
+            self.socket = Socket::STARTTLS;
+            self.port = match self.protocol {
+                Protocol::IMAP => 143,
+                Protocol::SMTP => 587,
+            };
+            res.push(self.clone());
+
+            // Try TLS
+            self.socket = Socket::SSL;
+            self.port = match self.protocol {
+                Protocol::IMAP => 993,
+                Protocol::SMTP => 465,
+            };
+            res.push(self);
+        } else if self.socket == Socket::Automatic {
+            // Try TLS over user-provided port.
+            self.socket = Socket::SSL;
+            res.push(self.clone());
+
+            // Try STARTTLS over user-provided port.
+            self.socket = Socket::STARTTLS;
+            res.push(self);
+        } else {
+            res.push(self);
+        }
+        res
+    }
+}