fileutil.write: accept mode=, and use it in Node.write_config

fileutil.write: accept mode=, and use it in Node.write_config

I want mode="w" (i.e. text, with newline conversion) for code that
writes newline-terminated strings (which should also be human readable)
to files. I like to use things like "cat .tahoe/permutation-seed"
without seeing the seed jammed together with the next command prompt.