selinux: introduce SECURITY_SELINUX_DEBUG configuration

The policy database code contains several debug output statements
related to hashtable utilization.  Those are guarded by the macro
DEBUG_HASHES, which is neither documented nor set anywhere.

Introduce a new Kconfig configuration guarding this and potential
other future debugging related code.  Disable the setting by default.

Suggested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
[PM: fixed line lengths in the help text]
Signed-off-by: Paul Moore <paul@paul-moore.com>