Major FUSE improvements

I think the shared writable mmap support will make happy many Wine and VMware users using
NTFS-3G who were waiting for this a few years to happen. Big thanks!

Apparently the large write support is also making a big impact on performance (much higher
bandwidth or much lower CPU usage). Previously the maximum write speed was around 100 MB/s,
now the highest ever measured is 802 MB/s which probably could be easily(?) improved by
lifting the current scalability limit at 128 kB block size. Some recent benchmark data

 block         
  size  tmpfs   ntfs-3g  ext3

   512     421      16      128
     1k     613      31      287
     2k     775      57      393
     4k     898      98      545
     8k     949     174      579
   16k     973     289      593
   32k     964     395      603
   64k     971     515      613
 128k     977     665      621
 256k     979     661      622
 512k     979     665      618
    1M     977     644      625