Allow to create new files (but changing them is not allowed yet)

- since mediafire cannot deal with empty files, we put new files into a
   temporary location and upload them once they get closed
 - add create and write functions to fuse
 - pass a file handle to mfconn_api_upload_simple instead of a path
 - allow calc_sha256 to also compute the file size
 - error out when the key returned by upload/simple is empty
 - make valgrind.supp more lenient

utils/hash.h

@@ -20,7 +20,8 @@
 #define _MFSHELL_HASH_H_
 
 int             calc_md5(FILE * file, unsigned char *hash);
-int             calc_sha256(FILE * file, unsigned char *hash);
+int             calc_sha256(FILE * file, unsigned char *hash,
+                            uint64_t * file_size);
 int             base36_decode_triplet(const char *key);
 void            hex2binary(const char *hex, unsigned char *binary);
 char           *binary2hex(const unsigned char *binary, size_t length);