mktables: Use Input_file class for always skipped files

mktables: Use Input_file class for always skipped files

Until this commit there were two mechanisms available to specify files
in the Unicode Character Database are not used by mktables.  Now there
is one.  The global that contained such files is deleted, and instead
all such files are specified by an Input_file class object.  This has
the advantage of just one method, and the constructor already has
parameters to specify when a file first appeared, and when it was
removed.  This allows automatic generation of the pod, listing just the
appropriate files for the version being compiled.  It also allows for
the automatic check of all files to see that they are DOS 8.3 filesystem
compatible.  And it allows for some code simplification.

Unicode specifies some .html files in the UCD.  These are always skipped
(so far, and likely forever), and were in the global.  Now they are in
the constructor, which means that the code that looks for potential
files that aren't being handled has to be changed to also look for .html
files as well.