Remove /File Import line length limit.

[taviso]

There is a line-length limit of 512 characters for /File Import. I think this is from fm_file_import(), I see no reason we can't just remove that limit, or at least make it really big.

[taviso]

I have a patch that fixes this, but I want to write some tests to make sure it works reliably.

It seems like the longest possible line you could ever want to import would be 512*256 = 132k (the longest possible label is 512 bytes, and there are 256 columns), but that seems ridiculously big. The current longest possible import line is 1024, which seems equally ridiculously, but too small.

Let's go for 4096 for now, and I'll increase it if anyone complains.