Replace cfile.c with connection.c

- cfile.h needed too many function calls and was too complex
 - connection.h does
     - only download in binary mode (json can handle that)
     - have no excess of getters and setters
     - allow to execute the whole request in a single function call
     - allow to be re-used for multiple requests
 - as a result, the code has 600 lines of code less
 - originally, connection.h was developed to use a global curl
   handle for all requests such that the same connection could be
   re-used. Unfortunately the MediaFire servers will close the
   connection after each request from their end:

      Bryan: "Unfortunately, we won't ever do keep-alive.  Closing the
      connection is a small part of a larger set of heuristics we have in
      place to prevent DOS/DDOS attacks."

   This causes massive performance impacts and those grow even larger when
   using SSL because the handshake has to be executed for every single
   request again.

private.h

@@ -22,11 +22,14 @@
 #define _MFSHELL_PRIVATE_H_
 
 #include <inttypes.h>
+#include <stdbool.h>
+#include <curl/curl.h>
 
 typedef struct _mfshell_s   _mfshell_t;
 typedef struct _folder_s    _folder_t;
 typedef struct _file_s      _file_t;
 typedef struct _cmd_s       _cmd_t;
+typedef struct _conn_s      _conn_t;
 
 struct _cmd_s
 {
@@ -59,6 +62,20 @@ struct _file_s
     char        *onetime_link;
 };
 
+struct _conn_s
+{
+    CURL       *curl_handle;
+    char       *write_buf;
+    size_t      write_buf_len;
+    double      ul_len;
+    double      ul_now;
+    double      dl_len;
+    double      dl_now;
+    bool        show_progress;
+    char        error_buf[CURL_ERROR_SIZE];
+    FILE       *stream;
+};
+
 struct _mfshell_s
 {
     int         app_id;