Use memset instead of vim_memset

Ran some commands on terminal, reviewed changes, and made some manual changes
too.

    find src | xargs perl -i -p -e "s/vim_memset/memset/g"
    git grep -l memset | egrep "\.c$" | xargs perl -i -p -e \
        's/(#include "vim\.h")/#include <string.h>\n\n\1/g'