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
Correct documentation of 'x' in kadm5.acl

It really is all privileges, including setting keys and
disallowing the propagation of the principal database.
Unlocked db2 KDB iteration

Try to make an optional db2 iterator that unlocks (and therefore
closes) the KDB between iterations, to allow large KDBs to be dumped
without blocking writes for a long time.

This is a hackish proof of concept.  It won't work for all consumers
of db_iterate, especially ones that need to hold other locks while
iterating.