Fix bug #6628 - "smbpassdb -a" using rid algorithm with tdbsam passdb

Fix bug #6628 - "smbpassdb -a" using rid algorithm with tdbsam passdb

samu_set_unix() does not use the rid allocator, but forces to use the
algoritmic allocator unconditionally, this is wrong and 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.)
(cherry picked from commit e1d4db8f55b25c7e9c408cb334fb89ccfd4c6565)

Signed-off-by: Michael Adam <obnox@samba.org>
This fixes bug #6628.
(cherry picked from commit 0a1b125ae4260de3cee6a92b19e3977d922a8c11)