Decode the 'who' argument passed to getrusage().

Add a new sysdecode_getrusage_who() which decodes the RUSAGE_* constant
passed as the first argument to getrusage(). Use this function in both
kdump and truss to decode the first argument to getrusage().

PR: 215448
Submitted by: Anton Yuzhaninov <citrin+pr@citrin.ru>
MFC after: 1 month