- avoid %nil to pass download_files service

- Update license tag to SPDX 1.2.

- Update to 8.6.2:
* TIP 429: New command [string cat].
* [lsearch -integer], [lsort -integer] operate on wide ints.
*** POTENTIAL INCOMPATIBILITY ***
* [dict replace], [dict remove] return canonical dicts.
*** POTENTIAL INCOMPATIBILITY ***
* Improved ::env synchronization with environment.
* transchans converted blocked writes to errors.
* [info class subclasses ::oo::object] includes ::oo::class .
* Fix: [chan configure -error] breaks [socket -async] connection.
* Better handle NUL in filenames.
* Fix: failures in nested ensemble bytecode compiler.
* Fixed crashes in...
- [apply {{} {while 1 {a {*}
[return -level 0 -code continue]}}}]
- [apply {{} {namespace upvar a b [x]}}]
- [coroutine X coroutine Y info frame]
- multi-interp font teardown
- oo chain deletion
* Plugged memory leaks in...
- execution traced coroutine deletion
- [close [zlib push ... -dictionary ...]]
- AtForkChild()
- managing oo instance lists
* Performance addressed in...
- [lappend l $multiple $arg] compile: quadratic -> linear