Use memset() instead of bzero() and memcpy() instead of bcopy(), there

Use memset() instead of bzero() and memcpy() instead of bcopy(), there
is no relevant difference for sbufs, and it increases portability of
the source code.

Split the actual initialization of the sbuf into a separate local
function, so that certain static code checkers can understand
what sbuf_new() does, thus eliminating on silly annoyance of
MISRA compliance testing.

Contributed by:		An anonymous company in the last business I
			expected sbufs to invade.