Work in progress restoring ability to [read] after [eof] and get non-empty
strings back in those cases where the channel has them to offer.  Also
working through all the implications of this possibility on Tcl's more
exotic channel features, like stacking.
merge 8.5
Base test for [5adc350683].
[5adc350683] Reworked the management of the EOF states to re-enable the
ability to read beyond EOF.  Plenty of assert()s to keep thing from going
off track again.
merge 8.5
New test iogt-7.0 demos bug in [testchannel transform].
fix failing test
Also test transfroms that delay.
[5adc350683] Stop Tcl forcing EOF condition on channels to be permanent.
It may be fleeting, and all parts of Tcl channel ecosystem have to deal
with that.  New assertions and tests to keep us on track.