Ipcs(1) POSIX equivalent to System V

POSIX does not specify a complete operating system, so any POSIX-compliant OS will have commands that aren't in POSIX (like init, mkfs, passwd, …). But different OSes have different extensions, and GNU tools (found on non-embedded Linux systems) have a lot.

BusyBox is a set of command-line tools that is intended for embedded Linux systems. It contains most of the utilities and options mandated by POSIX (it's not complete, but it comes close). You can make a running Linux system with a bootloader, a Linux kernel, BusyBox, and Dropbear if you want to log in over SSH. Add Gcc if you want to do development on the minimal system.

If you prefer to start from a full but small distribution, look at MINIX 3. This is a small unix system intended for embedded systems and for teaching.

If you want a more easily extensible system, look at OpenBSD. OpenBSD is focused on security and is conservative on features, but the core system does include major components such as Perl and Apache.

There is no command I know of but there exists a libc function call which can get the statistics:

man 3 mq_getattr

   mq_getattr() returns an mq_attr structure  in  the  buffer  pointed  by
   attr.  This structure is defined as:

       struct mq_attr {
           long mq_flags;       /* Flags: 0 or O_NONBLOCK */
           long mq_maxmsg;      /* Max. # of messages on queue */
           long mq_msgsize;     /* Max. message size (bytes) */
           long mq_curmsgs;     /* # of messages currently in queue */
       };