genfstab: get rid of `lsblk` and use `blkid` or `findmnt` instead

lsblk uses libudev to get device information, but udev isn't always present in the environment where genfstab is run. On the contrary, blkid uses libblkid and gets those informations directly, and shall be preferred to use.

For getting FSTYPE, findmnt should do.

Fixes #24 (closed) Closes #26