Looking at the backup dates for files in <mdl.int>, mdl106.exe is from
20th January 1981, whereas some of the source files are from a couple of
years later. Revert to the last version prior to 20th January 1981 -- in
every case, this was the earliest revision that was kept in <mdl.int>.
This undoes the changes that we'd previously made to these files, many
of which are no longer necessary now that we're using MIDAS 73.
On ITS, ILOOP in INITM fails because it runs out of address space;
it overruns by about 2000 words. There's a comment in MUDDLE suggesting
that BOT was originally 600000, so move it back to there (although we
could get away with a higher value if necessary in the future).
On TENEX, TXPURE would be used instead.
