Add OID for the vm.overcommit sysctl. This makes it possible to remove
one call to sysctl(2) from jemalloc startup code. (That also requires
changes to jemalloc, but I plan to push those to upstream first.)

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12745