use findmnt to look up mounts

[paralin]

 - Try to use findmnt to lookup mountpoint information.
 - If findmnt fails, fallback to the older approach.
 - Add no-resize-partition and ignore-resize-partition flags.

[paralin]

Supersedes #15

cc @ymorin-orange

used in Buildroot / SkiffOS: https://github.com/skiffos/buildroot/commit/adc5f3aba9a323ed92ee58935e65afc088c99f3c