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

Follow-up to r316179: More of the same

Reported by: Coverity
CIDs: 1368705, 1368706, 1368707, 1368710
Sponsored by: Dell EMC Isilon

t_msgsnd: Use msgsnd()'s msgsz argument correctly to avoid overflow

msgsnd's msgsz argument is the size of the message following the 'long'
message type. Don't include the message type in the size of the message
when invoking msgsnd(2).

Reported by: Coverity
CID: 1368712
Sponsored by: Dell EMC Isilon

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

Mechanically replace #if defined(__FreeBSD__) and #if defined(__NetBSD__) with
their #ifdef equivalents for everything changed in contrib/netbsd-tests. There
are some items from the vendor tree that use #if defined(__FreeBSD__) or
#if defined(__NetBSD__) which are being left alone

Requested by: bde, rpaulo
Sponsored by: EMC / Isilon Storage Division

Add limits.h #include for SSIZE_MAX

Sponsored by: EMC / Isilon Storage Division