I came across a test at
https://cs.android.com/android/platform/superproject/+/master:frameworks/native/libs/binder/tests/binderRpcTest.cpp;l=954;drc=68a556190553a4060babf4a4e5cb1bb16ae61ab2
that verifies that some fd passing code can handle passing SCM_MAX_FD
fds through a unix socket.  SCM_MAX_FD is an arbitrary 253 fd limit
imposed by the kernel since 2.6.38 (before that it was 255).  An
SCM_RIGHTS ancillary message contiang 253 fds is only slightly larger
than the existing 1024 byte internal buffer in sendmsg, so this patch
slightly increases the arbitrary limit in musl to match an arbitrary
limit in the kernel.