kern_sysctl: make sysctl.debug work as intended

r136999 introduced SYSTCL_DEBUG but apparently "opt_sysctl.h" was never
included making the option ignored.

r322954 introduced sysctl.reuse_test with OID number equal to 0, effectively
shadowing the very special sysctl.debug one. Use OID_AUTO as it doesn't need
any special treatment.

Reviewed by: kib (mentor)
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D23056