Fix MSVC warnings.

Some of these pointed to real potential overflows (given arbitrary
 inputs by the calling application).
I was sad about stripping const qualifiers from the struct addrinfo
 pointers, but MSVC seems to erroneously think that an array of
 pointers to constant data is itself a pointer to constant data (or
 maybe that it is not compatible with a const void *?), and
 converting the memmove()s to for loops triggered an erroneous
 warning about out-of-bounds array accesses in gcc (but on only one
 of the two identical loops).