increase sendmsg internal buffer to support SCM_MAX_FD

The kernel defines a limit on the number of fds that can be passed
through an SCM_RIGHTS ancillary message as SCM_MAX_FD. The value was
255 before kernel 2.6.38 (after that it is 253), and an SCM_RIGHTS
ancillary message with 255 fds requires 1040 bytes, slightly more than
the current 1024 byte internal buffer in sendmsg. 1024 is an arbitrary
size, so increase it to match the the arbitrary size limit in the
kernel. This fixes tests that are verifying they support up to
SCM_MAX_FD fds.