Check supplied hostname before using it.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
pkg: networkfs: etchosts: fixed tests

This patch fixes the fact that the tests for pkg/networkfs/etchosts
couldn't build due to syntax errors.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
integration-cli: docker_cli_links: fixed broken tests

The tests weren't ... tested when last edited, this patch fixes
them so that they run and pass correctly.

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
Remove support for MemoryReservation in systemd systems. This has been
deperecated since systemd 208.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Merge pull request #5626 from cyphar/networkfs-etchosts-tests

Fixed broken tests for pkg/networkfs/etchosts
Merge pull request #5627 from cyphar/integration-cli-fixes

Fixed completely broken integration-cli tests
Merge pull request #5629 from vmarmol/fix-systemd-softlimit

Remove support for MemoryReservation in systemd systems.
Merge pull request #5630 from rjnagal/libcontainer-fixes

Check supplied hostname before using it.