Document memset_s(3). memset_s(3) is defined in
C11 standard (ISO/IEC 9899:2011) K.3.7.4.1 The memset_s function
(p: 621-622)

Fix memset(3) portion of the man page by replacing the first argument
(destination) "b" with "dest", which is more descriptive than "b".
This also makes it consistent with the term used in the memset_s()
portion of the man page.

See also http://en.cppreference.com/w/c/string/byte/memset.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D13682