Wrap recvmsg() and sendmsg()

Wrap recvmsg() and sendmsg()

This introduces two new functions:

int socket_recvmsg(resource $socket, array &$msghdr, int $flags)
int socket_sendmsg(resource $socket, array $msghdr, int $flags)

The arrays representing struct msghdr follow the native counterpart
closely: structs are mapped to arrays, fields to array elements whose
key is the name of the field without the prefix (e.g. "name" instead
of "msg_name") and array are mapped to sequential numeric PHP arrays.

Right now the only type of ancillary data supported is fot the
level/type pair IPPROTO_IPV6/IPV6_PKTINFO.

I also refactored out the name resolution functions and made
sockets_strerror() a global function.