netbsd-tests: Remove some pointless sleeps from message queue tests

- In the msgctl tests, there is no point in sleeping after a fork().
Just block immediately in wait().
- In non-blocking send/recv tests, just wait for the child to exit once
it's reached a message limit. If a bug prevents the child from
exiting promptly, the test will time out.

MFC after: 1 week

Annotate all changes made in r316178-r316180 with __FreeBSD__

Restore the stock (upstream) code under an #else block, so it's easier
for me to visualize and understand the code that needs to be upstreamed.

MFC after: 2 months
X-MFC with: r316178, r316179, r316180
Sponsored by: Dell EMC Isilon

t_msgctl: Fix the same msgsnd() misuse as t_msgsnd

msgsnd(2)'s msgsz argument does not describe the full structure, only the
message component.

Reported by: Coverity
CIDs: 1368703, 1368711
Sponsored by: Dell EMC Isilon

Diff reduce with upstream by taking diff hunks that were contributed back

Add limits.h #include for INT_MAX

Sponsored by: EMC / Isilon Storage Division