Make PL/Tcl require Tcl 8.4 or later.

As of commit 287822068246a6ae30bb2c7191de727672ae6328, PL/Tcl will not
compile against pre-8.0 Tcl, whereas it used to work (more or less anyway)
with quite prehistoric versions.  As long as we're moving these goalposts,
let's reinstall them at someplace that has some thought behind it.  This
commit sets the minimum allowed Tcl version at 8.4, and rips out some bits
of compatibility cruft that are in consequence no longer needed.  Reasons
for requiring 8.4 include:

* 8.4 was released in 2002; there seems little reason to believe that
anyone would want to use older versions with Postgres 9.6+.

* We have no buildfarm members testing anything older than 8.4, and
thus no way to know if it's broken.

* We need at least 8.1 to allow enforcement of database encoding
security (8.1 standardized Tcl on using UTF8 internally, before that
it was pretty unpredictable).

* Some versions between 8.1 and 8.4 allowed the backend to become
multithreaded, which is disastrous.  We need at least 8.4 to be able
to disable the Tcl notifier subsystem to prevent that.

A small side benefit is that we can make the code more readable by
doing s/CONST84/const/g.

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