Add a function to convert from a disklabel p_fstype integer to a
dkw_ptype name.

branches: 1.3.2;
Add a version of proc_compare that works both for kernel and userland
(and uses the same algorithm!)

document non-literal format string

share with kernel; replaces bitmask_snprintf which was very broken:
- subtracted one from len without checking if len was 0
- checked for a minimum buffer size
- arguments order was inconsistent with other printf like functions
- no way to tell if it overflowed

sync with head