9pfuse with MacFUSE 4.x

[Connor Taffe]

I got around to trying FUSE out yesterday, and noticed that the newest version of MacFUSE is not supported. After a bit of mucking around, I discovered there were some differences from previous versions of osxfuse:

• vfs name is now macfuse
• Commands are now under /Library/Filesystems/macfuse.fs
• Command names are now load_macfuse and mount_macfuse
• Environment variables expected by mount_macfuse are now prefixed by _FUSE instead of MOUNT_OSXFUSE
• Mount now expects to return an fd across a socket similar to how it's handled in Linux, instead of passing the fd to the command. This was introduced in 3.3 but the old style has been removed in 4.x. Additionally, it expects the new environment variables _FUSE_COMMVERS=2 and _FUSE_CALL_BY_LIB=1 (without this it will say that mount_macfuse should be called only by the library and print help).

I've opened a PR with what worked for me: https://github.com/9fans/plan9port/pull/474.