samu_set_unix() does not use the rid allocator, but forces to use the

samu_set_unix() does not use the rid allocator, but forces to use the
algoritmic allocator unconditionally, this is wrong an inconsistent.
Use samu_alloc_rid_unix() instead.
(If we create a new user we should do it in a way consistent with all
other commands like pdbedit -a and net rpc user add.)