Fix cut-and-paste error message.

Imported from FreeBSD by patrick@
The, currently undocumented, -i option takes an argument.

Imported from FreeBSD by patrick@
dtc: don't crash if the argument is a directory.

Imported from FreeBSD by patrick@
dtc: ignore lines starting with #.

This is necessary because we use the C pre-processor to parse #include lines
and cpp adds line markings that start with #.

Imported from FreeBSD by patrick@