Create user btsync in DSM, for easy folder permissions

User is created with synouser, so people can give the btsync user comfortably folder permissions over DSM. The user is created with an random password and deactivated.

**Important:** New installation is needed, upgrading doesn't add the user in DSM.

Tested and working.

~~**Important 2:** Shit, you must deinstall the old version because, `synouser --del ${USER}` doesn't remove the user which was created with adduser. Should I add `deluser ${USER}` to preuninst?~~

~~Or should I create an new package? btsync-dsm?~~

~~**Testing:**~~
~~When people want to test it, here are the builds: http://www.andreasgiemza.de/btsync-for-synocommunity/1.4.83-create-user-in-dsm-test/~~

~~Old version must be removed first. ~~

~~When you have activated ssh. Please tell me the btsync line of /etc/passwd.~~

**Not working like it should ... when i change the folder permission of the btsync user the home directory and shell is reseted .. but only on my 413j. This is not happening on my 210j. That's a problem ...**