Docs for Tcl_CreateChannelHandler() state that the registered handler proc
will be called back with a mask value. "Mask is an integer mask indicating
which of the requested conditions actually exists for the channel; it will
contain ***a subset of the bits from the mask argument*** to
Tcl_CreateChannelHandler when the handler was created." (emhpasis added).

Tcl_NotifyChannel is not honoring this.  It passes a mask value that may
contain bits not in common with the mask argument to T_CCH().

This commit is a one-liner patch adding in the masking step to make things
behave as documented.  Thanks to apn for digging this out.  (In combination
with other questionable code, this led to a hang in test http-4.6 on Windows)

Tcl_NotifyChannel() has had this error in all of recorded Tcl history.
It's hard to imagine any code dependent on it though.  If any exists, it
can be revised to pass the mask value it truly needs to T_CCH() and end up
with code suitable both before and after this change.

If you concur, please merge to core-8-5-branch, and I'll take it from there.
Fix reviewed and accepted.