net: Implement register_net_sysctl.

Right now all of the networking sysctl registrations are running in a
compatibiity mode.  The natvie sysctl registration api takes a cstring
for a path and a simple ctl_table.  Implement register_net_sysctl so
that we can register network sysctls without needing to use
compatiblity code in the sysctl core.

Switching from a ctl_path to a cstring results in less boiler plate
and denser code that is a little easier to read.

I would simply have changed the arguments to register_net_sysctl_table
instead of keeping two functions in parallel but gcc will allow a
ctl_path pointer to be passed to a char * pointer with only issuing a
warning resulting in completely incorrect code can be built.  Since I
have to change the function name I am taking advantage of the situation
to let both register_net_sysctl and register_net_sysctl_table live for a
short time in parallel which makes clean conversion patches a bit easier
to read and write.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>