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.

ticket: 7924