fusefs: WIP making FUSE operations interruptible

The fuse protocol includes a FUSE_INTERRUPT operation that the client can
send to the server to indicate that it wants to abort an in-progress
operation. It's required to interrupt any syscall that is blocking on a
fuse operation.

This commit adds basic FUSE_INTERRUPT support. If a process receives any
signal while it's blocking on a FUSE operation, it will send a
FUSE_INTERRUPT and wait for the original operation to complete. But there
is still much to do:

• The current code will leak memory if the server ignores FUSE_INTERRUPT, which many do. It will also leak memory if the server completes the original operation before it receives the FUSE_INTERRUPT.
• An interrupted read(2) will incorrectly appear to be successful.
• fusefs should return immediately for fatal signals.
• Operations that haven't been sent to the server yet should be aborted without sending FUSE_INTERRUPT.
• Test coverage should be better.
• It would be great if write operations could be made restartable. That would require delaying uiomove until the last possible moment, which would be sometime during fuse_device_read.

PR: 236530
Sponsored by: The FreeBSD Foundation