load_file(): Treat fread() failures like other non-fatal file errors

load_file(): Treat fread() failures like other non-fatal file errors

... and check for underreads properly.

Previously Privoxy was supposedly serving the file partially
if it was edited in place, but actually would have served an
error message and leaked memory. Now it just serves the error
message (if it's run in a fantasy world were this actually
happens).

CID 66380, CID 66362, CID 66357.