sbcsgen.pl was giving different results on different machines in the case

sbcsgen.pl was giving different results on different machines in the case
where two SBCS code points mapped to a single Unicode point.
Changed so that by default it favours the lower SBCS code point.
On ixion, this highlighted ambiguities in CS_MAC_THAI, CS_MAC_SYMBOL, and
CS_VISCII. Guessed at a preference for the first two and added "sortpriority"
directives. (No idea about VISCII.)

[originally from svn r6641]
[this svn revision also touched charset,filter,halibut,timber]