Xattr tests build on OS X

Package syscall does not provide the Getxattr etc functions on OS X,
and native syscalls have a different signature anyway, so provide a
shim layer that looks the same on all platforms.
Statfs test builds on OS X now

Avoid using Statfs_t.Namelen, as OS X statfs reports no such field.

Don't use %q for displaying numbers.
Tests use sentinel errno ENAMETOOLONG now, for portability

EUCLEAN does not exist on OS X. ENAMETOOLONG was chosen by combination
of being fairly obscure and showing up in many syscall/zerrors_*.go
files in Go source.

Add a TODO note about the textual errno comparison.