Report from Rudolf Cejka <cejkar@fit.vutbr.cz> and a patch from him:

getopt(3) implementation under FreeBSD intorduces one more gloval variable, that
need to be initialized - optreset:

 extern int optreset;

     In order to use getopt() to evaluate multiple sets of arguments, or to
     evaluate a single set of arguments multiple times, the variable optreset
     must be set to 1 before the second and each additional set of calls to
     getopt(), and the variable optind must be reinitialized.

Apparently, rpcclient requires such a usage of getopt(), so the variable need to
be initialised.