netlink: initial netlink uevent monitor
utils: added var array list append and destroy
process: added ni_shellcmd_fmt_arg, cleaned up

- allow NULL as argument to ni_shellcmd_new
- added ni_shellcmd_fmt_arg formating one argument
- cleaned up reference counting functions and usage
Merge pull request #232 from mtomaschewski/wait-device-ready

netlink: initial uevent monitor