This allows us to build kernels which will (if the sysctl is enabled
too) to show pr_debug() and pr_devel() output.
Enable it by default for debug kernels.
This option can later also be used for other LinuxKPI parts, like
skbuffs, netdevice, rcu, ... to conditionally compile in debugging
support to further justify its existence beyond an in-code #define.
Sponsored by: The FreeBSD Foundation
MFC after: 3 days