Truncate logged Setxattr values

Normal values are output as hex, overly long values are output as hex
with a "..." suffix.
Test Setxattr with large values

Before bumping maxWrite to 16MB, TestSetxattr16MB would cause
/dev/fuse to close on OSX.
Ensure that all Requests implement the interface
Clarify RemoveRequest docs
Fix WriteFlags docs
Remove TODO note

Done as of a386bede5d9508af9c0f7c7ee516af8ea6bbc90f.
GetMountInfo describes a mounted file system, for tests
Mount options such as `-o fsname=foo` are supported via MountOptions

This commit implements FSName. More to come later.

fixes https://github.com/bazillion/fuse/issues/50
Support MountOption Subtype

This lets you identify your filesystem as type `fuse.foo` instead of
just `fuse`.
Remove burning hoops the error had to jump through

It works just the same with or without.
Share SimpleFS between tests
Share File and Dir helpers between tests
hellofs identifies as fuse.hellofs named helloworld
Support MountOption LocalVolume

fixes https://github.com/bazillion/fuse/issues/28
Support MountOption AllowOther, AllowRoot

Requested in https://github.com/bazillion/fuse/issues/50
Support MountOption VolumeName

Requested in https://github.com/bazillion/fuse/issues/50
Use factories for all MountOption types, for better godoc grouping