Release branch for Tk 8.4.20.
merge 8.4
 merge 8.4
merge 8.4
merge 8.4
merge 8.4
selected changes
Tag for release.
sync with Tcl's upgrade of dde/reg packages
merge-mark
merge 8.4
Fix for FreeBSD, and remove support for older FreeBSD versions. Patch by Pietro Cerutti
Fix for FreeBSD, and remove support for older FreeBSD versions. Patch by Pietro Cerutti
Fix for FreeBSD, and remove support for old FreeBSD versions. Patch by Pietro Cerutti
Fix for [3613668] by not calling XInitThreads when either 1) Tcl is compiled without threads 2) X11 library is too old.
A more direct test would be more desirable, but for now this should be fine.
Extra bit of safety.  Since we check whether xinitMutex is "still NULL"
to detect a threads-disabled Tcl, we should be explicitly certain it has
an initial value of NULL.
Disable XInitThreads() call until we have a better test for it.
Tag for release
docs typo
last-moment fix for FreeBSD from Pietro Cerutti