tests/unix_passfd: sending many and too many SCM_RIGHTS

o Exercise bounds checking when sending large set of file descriptors,

that can't fit into single control mbuf.

o Exercise resource limits checks when receiving a large sets.
o Check that socket isn't left in a completely stuck state when we can't

receive SCM_RIGHTS due to limits.  Current SOCK_STREAM socket would
free the control, but leave the data in.  This seems to be a legit
behavior for a stream socket, as we don't want holes in the data.

PR: 239250
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D35315