allow dockerd builds without cgo

[tonistiigi]

These patches allow building usable dockerd without the need of cgo. No changes to the current default way how we produce binaries.

Changes are in three areas:

Quota - uses cgo extensively but is an optional feature with feature detection already in caller. Add a new build tag that can be used to build without it.

Copy - cgo used for the ioctl based fast copy that has been replaced with the syscall. Change is to skip this check on !cgo . The copy_file_range and fallback methods still provide the feature. As it is only used for constant I could also just hardcode it in !cgo version if we want to keep it.

Stats: uses sysconf. Replaced with unix.SchedGetaffinity

[codecov]

Codecov Report

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The diff coverage is 100%.

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[thaJeztah]

NB: too bad we don't have a place to document all docker build tags :-\

@kolyshkin we could update https://github.com/moby/moby/blob/master/project/PACKAGERS.md#docker_buildtags

But in general we need to unify / cleanup project documentation

[yongtang]

LGTM