Add the who protocol (rwho/rwhod/ruptime)

Add the who protocol (rwho/rwhod/ruptime)

In packet_hex_print(), compute (bstart + blen) only once.

In time_secs_to_str(), return a meaningful string when time == 0, instead
of returing pointer to char buffer with old, inappropriate data in it.

svn path=/trunk/; revision=1297