wsrep_sst_rsync.sh: apply safe() to joiner-supplied parameters

Issue:
wsrep_sst_rsync.sh interpolated WSREP_SST_OPT_REMOTE_USER and
WSREP_SST_OPT_REMOTE_PSWD verbatim. Because both values originate from
the joiner side of the SST request, a newline in either could splice
an extra directive into the donor-written stunnel.conf (silently
downgrading peer-cert verification) or an extra line into the rsync
magic file. MDEV-39413 had introduced safe() for the same threat class
in wsrep_sst_mariabackup but did not extend it to the rsync script.

Solution:
Routing the rsync interpolations through safe() closes the gap, and
extending safe() to also reject tab and newline ensures multi-line
values cannot survive into a config-file heredoc.