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

valgrind.supp

@@ -116,7 +116,6 @@
 {
    fuse_opt_parse
    Memcheck:Leak
-   match-leak-kinds: definite
    fun:malloc
    fun:strdup
    obj:*libfuse.so*