typedef_ask needs upstreaming

[ntyni]

U/typedefs/Typedef.U has a variable called 'typedef_ask', which seems to be a modified version of 'typedef' that prompts for a value if it can't be found otherwise. This comment in the file looks relevant:

?X:     Originally, I eval'd $typedef and then checked the results against
?X:     $type.  Unfortunately, it then became complicated to check
?X:     whether you are re-using an old config.sh value, and the logic got
?X:     so convoluted it became easier to just copy the function above
?X:     and alter it slightly.  A.D.  3/1998

There are seven units using typedef_ask, and it looks like at least U/typedefs/{modetype,pidtype,sizetype,fpostype}.U differ from upstream dist only because of this.

AFAICS the four units above implement the same prompting logic separately, so upstreaming this would also simplify upstream code.

I'm happy to look at forwarding this later when I have the time, but filing this to document the findings so far. Please comment in case I've missed something.