io.c: add Read_byte_line() function

src/io.h

@@ -44,6 +44,8 @@ int Write_byte(FILE *file, byte b);
 
 /// Reads several bytes from an open file. Returns true if OK, false if a file i/o error occurred.
 int Read_bytes(FILE *file, void *dest, size_t size);
+// Read a line from an open file. Returns true if OK, false in case of error
+int Read_byte_line(FILE *file, char *line, size_t size);
 /// Writes several bytes to an open file. Returns true if OK, false if a file i/o error occurred.
 int Write_bytes(FILE *file, void *dest, size_t size);