This change uses a reader count instead of holding the mutex for the
interpreter list to avoid the problem of holding a non-sleep lock during
a page fault as reported by witness. In addition, it consistently uses
memset()/memcpy() instead of bzero()/bcopy() except in the case where
bcopy() is required (i.e. overlapping copy).

Differential Revision: https://reviews.freebsd.org/D2123
Submitted by: sson
MFC after: 2 weeks
Relnotes: Yes