vfs_fruit: Add capability to advertise FULLSYNC

[github at samba.org]

New comment by satmandu on Samba Github repository

https://github.com/samba-team/samba/pull/64#issuecomment-385246504
Comment:
That's as expected. 18.04 was in pre-release lockdown by the time samba 4.8.0 was released.

Use at your own risk, but for what it is worth, adding debian experimental as an apt source in 18.04 and installing samba using apt from debian experimental seems to work just fine.