allow to use host database connection parameter

Add config option:
db_host (default empty) - name of host to connect to

winixd/core/config.h

@@ -204,12 +204,37 @@ public:
 
 	// the database connection string
 	// https://www.postgresql.org/docs/14/libpq-connect.html#LIBPQ-CONNSTRING
-	// default: empty (not used if empty)
+	// default: empty
+	// if empty then winix uses db_host and db_hostaddr
 	std::wstring db_conn_string;
 
-	// the database host ip address and port number
-	// default: empty which means winix connects to a Unix-domain socket
+	// the database host name used if db_conn_string is empty
+	// default: empty
+	std::wstring db_host;
+
+	// the database host ip address used if db_conn_string is empty
+	// default: empty
+	// if db_host is empty and db_hostaddr is empty then winix connects to a Unix-domain socket
+	// meaning of db_host and db_hostaddr parameters is the same as described in
+	// https://www.postgresql.org/docs/14/libpq-connect.html#LIBPQ-CONNSTRING
+	//
+	// from above documentation:
+	// Using hostaddr allows the application to avoid a host name look-up, which might be important
+	// in applications with time constraints. However, a host name is required for GSSAPI or SSPI
+	// authentication methods, as well as for verify-full SSL certificate verification.
+	// The following rules are used:
+	//
+	// - If host is specified without hostaddr, a host name lookup occurs.
+	//
+	// - If hostaddr is specified without host, the value for hostaddr gives the server network address.
+	//   The connection attempt will fail if the authentication method requires a host name.
+	//
+	// - If both host and hostaddr are specified, the value for hostaddr gives the server network address.
+	//   The value for host is ignored unless the authentication method requires it, in which case it will
+	//   be used as the host name.
 	std::wstring db_hostaddr;
+
+	// the database port number
 	std::wstring db_port;
 
 	// the database name, user name and a password for the PostgreSQL database