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