Added constants: SCM_RIGHTS, SCM_CREDENTIALS and SO_PASSCRED.
The function socket_cmsg_space() was modified to support message types with
variable size. Its new signature is:
int socket_cmsg_space(int $level, int $type, int $n)
where $n is the number of repetable elements that the message is composed of.
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.
