Update ds18b20.c

Original ds18b20.c seems to support only the 'w1-gpio', here I have added support for Dallas OWFS (one wire file system by dallas) 

I have my 1-wire stuff attached to I2C 1 wire 8-channel master controller DS2482-800 (with 1 channel DS2482-100 it should work also) which works with Dallas OWFS (one wire file system) and here there are a little differences:
1. The path is to be choosen, I choosen /mnt/1wire/
(it is impossible to use /sys/bus/w1/devices/)
2. The sensor names can include big letters
3. There is '.' instead '-' in sensors names ("28.1C2A93050000" instead "28-1c2a93050000")
4. The temperature can be read from file named 'temperature'

For example:
/mnt/1wire/28.1C2A93050000/temperature