Change several memcpy() to memmove() in cases when the regions are
definitely overlapping; memcpy() of overlapping regions is undefined
behavior in C and can produce different results depending on the compiler,
the memcpy implementation, etc.

Cc: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>