This also accomplishes the following:
- Proxy through zfs_nicenum as be_nicenum, because it looks better than
humanize_number and would presumably be useful to other libbe consumers.
- Rename be_get_snapshot_props to be_get_dataset_props, make it more useful
At a bare minimum, this function will return 0 if a BE is mounted at the
given path or non-zero otherwise. If the optional 'details' nvlist is
supplied, it is filled with an nvpair containing just the information about
the BE mounted at the path. This nvpair is structured just as it is for
be_get_bootenv_props, except limited to just the single mount point.
Based on the idea that we shouldn't have all-new library and utility going
into base that need WARNS=1...
- Decent amount of constification
- Lots of parentheses
- Minor other nits
