OPENAFS-SA-2018-003 rxgen: prevent unbounded input arrays

OPENAFS-SA-2018-003 rxgen: prevent unbounded input arrays

RPCs with unbounded arrays as inputs are susceptible to remote
denial-of-service (DOS) attacks.  A malicious client may submit an RPC
request with an arbitrarily large array, forcing the server to expend
large amounts of network bandwidth, cpu cycles, and heap memory to
unmarshal the input.

Instead, issue an error message and stop rxgen when it detects an RPC
defined with an unbounded input array.  Thus we will detect the problem
at build time and prevent any future unbounded input arrays.

Change-Id: Ib110f817ed1c8132ea2549025876a5200c728fab