Fix OpenFile flags to avoid garbled files

[jcaamano]

Reviewed and corrected OpenFile flags:

• added flag os.O_TRUNC to prevent leftovers at the end of exisitng files when new data does not overflow existing data.
• added flag os.O_CREATE to create file if is does not exist as is the most reasonable thing to do other than exceptions
• changed flag os.O_RDWR to os.O_RDONLY for files that are only read from.