Fix registry auth by storing the string passed on the command line, and allowing for credential selection by normalizing on hostname. Also, remove remote ping calls from CmdPush and CmdPull.

Docker-DCO-1.1-Signed-off-by: Jake Moshenko <jake@devtable.com> (github: jakedt)
Fix minor typo
handle symlinks for Docker's root dir & TMPDIR

This removes the incomplete symlink handling from engine.go and it adds
it one place in docker.go.

It also enables handling symlinks for TMPDIR.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #4382 from unclejack/fix_symlink_handling

handle symlinks for Docker's root dir & TMPDIR
Merge pull request #4270 from DevTable/fixregistryauth

Fix registry auth and remove other hidden ping commands from client code
Update daemon docs for exec drivers
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4415 from iantruslove/patch-1

Fix minor typo
Merge pull request #4438 from crosbymichael/update-docs

Update daemon docs for exec drivers