MFC r316179,r316180,r316181,r316260:

r316179 (by cem):

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).

CID: 1368712

r316180 (by cem):

Follow-up to r316179: More of the same

CIDs: 1368705, 1368706, 1368707, 1368710

r316181 (by cem):

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.

CIDs: 1368703, 1368711

r316260:

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.