ksplice: add sysctls for determining Ksplice features.

ksplice: add sysctls for determining Ksplice features.

This adds a new set of sysctls that the Ksplice user-space can use for
feature tests.  The first sysctl added is ksplice.init_trace_safe which
means that it is safe to trace a child_subreaper processes without
making them killable which was fixed in 2d39b3cd34 (signal: protect
SIGNAL_UNKILLABLE from unintentional clearing.).

Ksplice can now test this sysctl and use that to determine whether it is
safe to trace init or not.  Future sysctls can be added for other
features, as well as kernel internal features for kernel Ksplice.

Orabug: 25414814

Signed-off-by: Jamie Iles <jamie.iles@oracle.com>
Reviewed-by: Dhaval Giani <dhaval.giani@oracle.com>
Conflicts:
	kernel/Makefile