[nfs] bump to libnfs 1.6

Fixes issues with updated kernel nfs server where gid = -1 is invalid as of now. This affects ios/atv2/osx/android/linux (in ppa the version bump was already done).

@gimli could you test compile it for rbpi please? Also how much work would it be to get rid of tools/rbpi/depends and use unified depends for rbpi aswell?

libnfs 1.6 bump for windows will be seperate since it first needs upstream compilation fixing.