[krbdev.mit.edu #7924] tcl_kadm5.c is incompatible with Tcl 8.6

[Greg Hudson via RT]

Tcl has long deprecated the direct use of the "result" field of 
Tcl_Interp.  In 8.6 the field becomes inaccessible unless 
USE_INTERP_RESULT is defined, and generates deprecation warnings even if 
it is.  Supposedly 9.0 will remove the field altogether.

The modern idiom is Tcl_SetResult.  Since we currently sprintf into 
interp->result, we probably want to make a helper function which calls 
snprintf into a buffer and then calls Tcl_SetResult with TCL_VOLATILE.