Make tcl_kadm5.c work with Tcl 8.6

Directly accessing the result field of Tcl_Interp has been deprecated
for a long time, requires a special define in Tcl 8.6, and will be
impossible in Tcl 9.  Use Tcl_SetResult instead.  The new error
messages are less helpful than the old ones, but this is just support
infrastructure for old tests, so it isn't important.

(cherry picked from commit b63496d7b44f090ea5d300dc09b4fc043138ae38)

ticket: 7924
version_fixed: 1.12.2
status: resolved