fixes #73

this allows us to distinguish between truncate vs ftruncate

previously, this will fail if /mnt/file doesn't have an xattr:

```
listxattr("/mnt/file", 0x7fe8b3686830, 256) = -1 EIO (Input/output error)
```

We should be returning the actual size only if the input size is
zero. Related issue is if the filesystem returns ERANGE, we should
propagate that error instead of returning the actual size.

Replaced go-xattr usage with x/sys/unix so we can test this.

* Fallocate support

* use fallocate wrapper that works on darwin

try mounting without fusermount(1) first

/dev/fuse is not pollable. Go ≥ 1.13 starts reporting this on Read().

This requires root privileges or the CAP_SYS_ADMIN capability, but allows
supporting -o suid.

See https://golang.org/doc/go1.13#os for the corresponding change.

replace imports of "golang.org/x/net/context" with "context"

Hard links implementation

add fuse mount subtype to mountconfig

when we are under memory pressure, or echo 3 > /proc/sys/vm/drop_caches,
kernel can send us many forget ops at the same time if we have lots of
inodes in memory. Running them all in goroutines can lead to even more
memory usage and eventually OOM.

Closes #21.

See #25.

MacOS apparently invokes StatFS on os.Lstat/os.Stat. Note that the
return fields are not actually populated here.