Consistently capitalize CLI options

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Made CLI option capitalization consistent in api/client.go

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Updated a variety of language and error messages

In api/client.go updated:

1. Made api == API
2. Restated Docker daemon connection error to be clearer.
3. Changed use of "Impossible" in error messages.
4. Removed extra space in restart error message.
5. Fixed capitalization in three error messages.

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
libcontainer: Use MS_PRIVATE instead of MS_SLAVE

Now that we unmount all the mounts from the global namespace we can
use a private namespace rather than a slave one (as we have no need
for unmounts of inherited global mounts to propagate into the
container).

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Create pkg/system and move stuff there from archive

This is a package for generic system calls etc that for some reason
is not yet supported by "syscall", or where it is different enough
for the different ports to need portability wrappers.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4465 from alexlarsson/private

libcontainer: Use MS_PRIVATE instead of MS_SLAVE
Merge pull request #4278 from alexlarsson/system

Create pkg/system and move stuff there from archive
Merge pull request #4461 from jamtur01/capital

Consistently capitalize CLI options
Merge pull request #4462 from jamtur01/langfix

Updated a variety of language and error messages