Support forget inode ops

[jacobsa]

We define a ForgetInodeOp, but don't currently ever issue it. Start issuing it. Things:

• Give up on the idea of reference counting within the fuse package and just send the count through to the user. They can naturally track counts in their inode structure.
• Be very precise in the documentation about what increments the count.

[jacobsa]

Still to do:

• Write a file system called forgetfs. Same structure as flushfs. It maintains the reference count according to the documentation. It has a public method that panics if any inodes are left with a reference count. It also panics on negative reference count.
• Write a test that exercises various methods of forgetfs. TearDown unmounts as usual and then calls the check method.