Add the `name converter` daemon parameter.

Add the `name converter` daemon parameter.

This is based on the long-standing patch but with the protocol changed
to just use newlines as delimiters instead of null chars (since names
should not contain a newline AND it makes it easier to write a helper
script).  Lots of other small improvements and a better default value
for "numeric ids" when using "use chroot" with "name converter".