libc headers: make <complex.h> work, add struct ipv6_mreq (#1100)

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.

Author: dfyz

libc/complex.h

@@ -1,5 +1,5 @@
-#ifndef _COMPLEX_H
-#define _COMPLEX_H
+#ifndef COSMOPOLITAN_LIBC_COMPLEX_H_
+#define COSMOPOLITAN_LIBC_COMPLEX_H_
 COSMOPOLITAN_C_START_
 #if __STDC_VERSION__ + 0 >= 201112 && !defined(__STDC_NO_COMPLEX__)
 
@@ -116,4 +116,4 @@ complex long double cpowl(complex long double, complex long double) libcesque;
 
 #endif /* C11 */
 COSMOPOLITAN_C_END_
-#endif /* _COMPLEX_H */
+#endif /* COSMOPOLITAN_LIBC_COMPLEX_H_ */

libc/isystem/netdb.h

@@ -5,6 +5,7 @@
 #include "libc/sock/struct/in6_pktinfo.h"
 #include "libc/sock/struct/in_pktinfo.h"
 #include "libc/sock/struct/ip_mreq.h"
+#include "libc/sock/struct/ipv6_mreq.h"
 #include "libc/sock/struct/sockaddr.h"
 #include "libc/sock/struct/sockaddr6.h"
 #include "libc/sysv/consts/in.h"

libc/isystem/netinet/in.h

@@ -6,6 +6,7 @@
 #include "libc/sock/struct/in6_pktinfo.h"
 #include "libc/sock/struct/in_pktinfo.h"
 #include "libc/sock/struct/ip_mreq.h"
+#include "libc/sock/struct/ipv6_mreq.h"
 #include "libc/sock/struct/sockaddr.h"
 #include "libc/sock/struct/sockaddr6.h"
 #include "libc/sysv/consts/in.h"

libc/sock/struct/ipv6_mreq.h

@@ -0,0 +1,12 @@
+#ifndef COSMOPOLITAN_LIBC_SOCK_STRUCT_IPV6_MREQ_H_
+#define COSMOPOLITAN_LIBC_SOCK_STRUCT_IPV6_MREQ_H_
+#include "libc/sock/struct/sockaddr6.h"
+COSMOPOLITAN_C_START_
+
+struct ipv6_mreq {
+  struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast address of group */
+  unsigned ipv6mr_interface;        /* local interface */
+};
+
+COSMOPOLITAN_C_END_
+#endif /* COSMOPOLITAN_LIBC_SOCK_STRUCT_IPV6_MREQ_H_ */