fusefs: merge from projects/fuse2

fusefs: merge from projects/fuse2

This commit imports the new fusefs driver. It raises the protocol level
from 7.8 to 7.23, fixes many bugs, adds a test suite for the driver, and
adds many new features. New features include:

* Optional kernel-side permissions checks (-o default_permissions)
* Implement VOP_MKNOD, VOP_BMAP, and VOP_ADVLOCK
* Allow interrupting FUSE operations
* Support named pipes and unix-domain sockets in fusefs file systems
* Forward UTIME_NOW during utimensat(2) to the daemon
* kqueue support for /dev/fuse
* Allow updating mounts with "mount -u"
* Allow exporting fusefs file systems over NFS
* Server-initiated invalidation of the name cache or data cache
* Respect RLIMIT_FSIZE
* Try to support servers as old as protocol 7.4

Performance enhancements include:

* Implement FUSE's FOPEN_KEEP_CACHE and FUSE_ASYNC_READ flags
* Cache file attributes
* Cache lookup entries, both positive and negative
* Server-selectable cache modes: writethrough, writeback, or uncached
* Write clustering
* Readahead
* Use counter(9) for statistical reporting

PR:		199934 216391 233783 234581 235773 235774 235775
PR:		236226 236231 236236 236291 236329 236381 236405
PR:		236327 236466 236472 236473 236474 236530 236557
PR:		236560 236844 237052 237181 237588 238565
Reviewed by:	bcr (man pages)
Reviewed by:	cem, ngie, rpokala, glebius, kib, bde, emaste (post-commit
		review on project branch)
MFC after:	3 weeks
Relnotes:	yes
Sponsored by:	The FreeBSD Foundation
Pull Request:	https://reviews.freebsd.org/D21110