Fixes #610 and #612 - two telnet terminal issues.

[Dunbaratu]

Problem #610:
The problem was caused by the screen differ attempting
to actually print the nullchars stored in the screen
buffer instead of printing them as space chars. Thus a
diff chunk that should have been printed as "AAA BBB" got
printed out as "AAA" followed by 3 null chars, followed by
"BBB". The terminal on the client end will not advance the
cursor forward when printing a null char, thus the "BBB" got
printed adjacent to the "AAA".

Interestingly, the reason I didn't detect the problem in my own testing
before release is that it need exactly the following conditions to be true
in order to trigger it:

(A) the area of the screen being PRINT AT'ed to has to have been
    previously unused. (If it had been populated by some text and then
    the text overwritten by  the PRINT AT, then the PRINT AT worked
    fine.  For example, if you print out a decorative border with spaces
    filling its inside, then paint the values atop those spaces, the bug
    didn't appear - which is why I didn't catch it.)
and
(B) The run of space between the "AAA" and the "BBB" has to be no
    larger than JOIN_DIFF_DIST (6) to trigger the problem, thus
    PRINT "AAAAAA" AT (20,5). PRINT "BBB" AT (30,5).
    will trigger the problem, while
    PRINT "AAAAAA" AT (20,5). PRINT "BBB" AT (40,5).
    will not.  If the two strings are far enough apart, then the
    system will teleport the cursor from one to the other instead
    of trying to print out the intervening space.

Problem #612:
The problem was caused by slight messy behavior about the
timestamp ages on rows of the screen that are occluded by a
subbuffer and then the subbuffer shrinks to reveal more of
the screen it was masking. The system didn't mark the screen
rows it previously masked off as being newly changed and in
need of an update.