Check for MON_IOCH_MFLUSH failing.

It shouldn't fail, but, just in case it does, report the error.

Failure to check the return value was noticed by Coverity.
Cast away return values if we're not checking them.

When destroying a ring buffer, there's not much we can do if the call to
destroy the ring buffer fails, so we just ignore the return value of
setsockopt(); cast it to void, to explicitly indicate that we're
ignoring it, in the hopes that Coverity will no longer complain that
we're ignoring it.  Add a comment noting this.

Do the same with the return value of munmap(), as we're ignoring it for
the same reason (even though Coverity, for some unknown reason, didn't
complain about it).
Add some more error checking.

Most of these are "should not fail" cases, but let's at least report
their failures as fatal errors.
Fix copy-and-pasteos.