mp_rpc_server.C: limit iteration of svc_fdset to FD_SETSIZE, as per spec.

mp_rpc_server.C: limit iteration of svc_fdset to FD_SETSIZE, as per spec.

In init() there was code iterating over all of the possible file
descriptors in a svc_fdset.  fdsets are limited to FD_SETSIZE.  This
caused coredumps on FreeBSD 10, and possibly other hidden issues.

Moving to poll(), rather than select() would be better, but is a bigger
job.  For now, just limit to the FD_SETSIZE that select() requires.