1. `libc/isystem/complex.h` (included when you do `#include <complex.h>`)
defines `_COMPLEX_H`, and then proceeds to include `libc/complex.h`,
which contains the actual complex-related declarations. However, they
are *also* guarded by `_COMPLEX_H` and hence effectively ignored.
Fix this by changing `_COMPLEX_H` to `COSMOPOLITAN_LIBC_COMPLEX_H_`,
which is consistent with what the other headers (such as `math.h`) do.
2. Cosmopolitan could only support IPv4 multicast requests for sockets,
since a declaration for `struct ipv6_mreq` was missing. Add support
for IPv6, too, by adding the missing declaration.
