Use memcpy helpers and FILLZERO

Use memcpy helpers and FILLZERO

Replace various calls to memcpy and memset with equivalent calls to
these macros.

There are still a couple of open-coded memcpy(,buf_unprepend(),)
in udp.c's proxy code which will be done later.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>