LU-17497 obdclass: check upcall incorrect values

LU-17497 obdclass: check upcall incorrect values

Identity upcall is set via lctl set_param mdt.*.identity_upcall=xxx,
and rsi upcall is set via lctl set_param sptlrpc.gss.rsi_upcall=xxx.
Possible values are a valid path to an executable, and also NONE to
disable identity upcall.
Add an upcall cache function that checks the user provided string, to
make sure we do not store an invalid value. And print a message to
stdout to explain the accepted values.

Add sanity-sec test_69 to exercise this.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Iaf59e72aa1612f5579db175d8999dcf0053308ed
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53878
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>