Calls the System V IPC function msgctl(2). You'll probably have to say
- use IPC::SysV;
first to get the correct constant definitions.  If CMD is IPC_STAT
,
then ARG must be a variable that will hold the returned msqid_ds
structure.  Returns like ioctl: the undefined value for error,
"0 but true"
 for zero, or the actual return value otherwise.  See also
SysV IPC in perlipc and the documentation for IPC::SysV
 and
IPC::Semaphore
.
Portability issues: msgctl in perlport.