Rename sysctl_lock and _unlock to sysctl_xlock and _xunlock.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273400 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Implement shared locking for sysctl.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273401 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f