Fix Lchmod() for Linux+Go1.11

[kolyshkin]

This fixes the bug that Go 1.11 uncovered (rendering driver.Lchmod() useless as it always returns an error now. Found during moby rebase to go-1.11beta1 ( https://github.com/moby/moby/pull/37358#issuecomment-404032896).

This set also optimizes and cleans up some things here and there. Please see individual commit messages for all the details.

The initial 3 commits are preparations:

• [PATCH 1/5] sysx/xattr_darwin.go: rm duplicate Fchmodat def
• [PATCH 2/5] sysx.Fchmodat(): unify remove implementations
• [PATCH 3/5] Lchmod(): simplify and optimize

The 4th commit is the fix itself:

• [PATCH 4/5] Lchmod(): fix for Linux/Go 1.11

The last one is a small post-fix followup:

• [PATCH 5/5] context.Apply: no need to skip chmod on symlinks

[kolyshkin]

Patchset updated to remove sysx.Fchmodat() entirely and use golang.org/x/sys/unix directly as suggested by @dmcgowan. This looks way better.

[dmcgowan]

LGTM

[crosbymichael]

LGTM