1.9.0-devel: _ in read.delim and make.names

In R 1.9.0, make.names will accept "_" as a valid character
for a syntactically valid name.

I would appreciate to have an option in ``read.delim'' (etc.)
that would change "_" in headers of input files to "."
for compatibility with code and data written for R 1.8.1 and before.

Wolfram Fischer