Remove panic in lxc driver.

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
devmapper: Enable skip_block_zeroing

This makes the device mapper not zero out blocks allocated on the
thinp device. This is safe in our use case, as we access the device
via a filesystem that doesn't leak any uninitialized data to userspace.

This partially helps with https://github.com/dotcloud/docker/issues/3280
and should generally improve preformance on the devicemapper backend.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Remove linux specific calls

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Add slightly better GOPATH detection/handling

This also adds a new "AUTO_GOPATH" environment variable that will create an appropriate GOPATH as part of the build process.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4013 from creack/remove_panic_lxc

Remove panic in lxc driver.
Merge pull request #4031 from tianon/gopath

Add slightly better GOPATH detection/handling
Merge pull request #4017 from alexlarsson/skip_block_zeroing

devmapper: Enable skip_block_zeroing
Improve no command handling
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4042 from crosbymichael/allow-empty-cmd

Improve no command handling
Merge pull request #3983 from creack/remove_linux_specific

Remove linux specific calls
Add better ".git" detection and use

This way, packagers can set GIT_DIR appropriately if they'd prefer to not have ".git" inside their working directory.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)