This change replaces the mutex with a sx lock for the interpreter list to
avoid the problem of holding a non-sleep lock during a page fault as
reported by witness. It also uses atomics where possible to avoid having
to acquire the exclusive lock. In addition, it consistently uses
memset()/memcpy() instead of bzero()/bcopy().

Differential Revision: https://reviews.freebsd.org/D1971
Submitted by: sson
Reviewed by: jhb