sysctl: Add register_sysctl for normal sysctl users

The plan is to convert all callers of register_sysctl_table
and register_sysctl_paths to register_sysctl.  The interface
to register_sysctl is enough nicer this should make the callers
a bit more readable.  Additionally after the conversion the
230 lines of backwards compatibility can be removed.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>