Signed-off-by: Rob Murray <rob.murray@docker.com>

Signed-off-by: Rob Murray <rob.murray@docker.com>

Until now it's been possible to set per-interface sysctls using, for
example, '--sysctl net.ipv6.conf.eth0.accept_ra=2'. But, the index in
the interface name is allocated serially, and the numbering in a container
with more than one interface may change when a container is restarted.
The change to make it possible to connect a container to more than one
network when it's created increased the ambiguity.

This change adds label "com.docker.network.endpoint.sysctls" to the
DriverOpts in EndpointSettings. This option is explicitly associated
with the interface.

Settings in "--sysctl" for "eth0" are migrated to DriverOpts.

Because using "--sysctl" with any interface apart from "eth0" would have
unpredictable results, it is now an error to use any other interface name
in the top level "--sysctl" option. The error message includes a hint at
how to use the new per-interface setting.

The per-endpoint sysctl name has the interface name replaced by
"IFNAME". For example:
    net.ipv6.conf.eth0.accept_ra=2
becomes:
    net.ipv6.conf.IFNAME.accept_ra=2

The value of DriverOpts["com.docker.network.endpoint.sysctls"] is a
comma separated list.

Settings from '--sysctl' are applied by the runtime lib during task
creation. So, task creation fails if the endpoint does not exist.
Applying per-endpoint settings during interface configuration means the
endpoint can be created later, which paves the way for removal of the
SetKey OCI prestart hook.

Unlike other DriverOpts, the sysctl label itself is not driver-specific,
but each driver has a chance to check settings/values and raise an error
if a setting would cause it a problem - no such checks have been added
in this initial version. As a future extension, if required, it would be
possible for the driver to echo back valid/extended/modified settings to
libnetwork for it to apply to the interface. (At that point, the syntax
for the options could become driver specific to allow, for example, a
driver to create more than one interface).

Signed-off-by: Rob Murray <rob.murray@docker.com>