I wrote a tiny C program (will email you the binary as an attachment in another email). It simply opens the file for APPEND in binary mode and a mode that allows reading as well as writing. I goto the end of the file, read the last character. If that is NOT '\n', I put a '\n' at the end of the file. If it is a '\n', I do nothing at that point (file is unchanged).

The source code is:

#include "stdio.h"

void main(int argc, char * argv[])
{
FILE * input;


if ( argc < 2 )
{
printf( "usage: addnl FILENAME\n" );
exit(1);
}

/* Append in binary mode, allow for reading */
input = fopen( argv[1], "a+b" );

if ( input == NULL )
{
printf( "Error opening file %s\n", argv[1] );
perror( "fopen()" );
exit(1);
}

/* goto last character */
fseek( input, -1, SEEK_END );

if ( fgetc( input ) != '\n' )
{
printf( "Adding newline, was not there\n" );
fseek( input, 0, SEEK_END );
fputc( '\n', input );
}
else
{
printf( "Newline detected at EOF, not added\n" );
}
fclose( input );
}

and it should be relatively fast as it does not rewrite the entire file, just fixes it up in place. I'll send you the full source, binary and makefile.