Relnotes: yes (subtle output/behavior change)

Relnotes:	yes (subtle output/behavior change)

MFC r316938,r316953:

r316938:

savecore: fix space calculation with respect to `minfree` in check_space(..)

- Use strtoll(3) instead of atoi(3), because atoi(3) limits the
  representable data to INT_MAX. Check the values received from
  strtoll(3), trimming trailing whitespace off the end to maintain
  POLA.
- Use `KiB` instead of `kB` when describing free space, total space,
  etc. I am now fully aware of `KiB` being the IEC standard for 1024
  bytes and `kB` being the IEC standard for 1000 bytes.
- Store available number of KiB in `available` so it can be more
  easily queried and compared to ensure that there are enough KiB to
  store the dump image on disk.
- Print out the reserved space on disk, per `minfree`, so end-users
  can troubleshoot why check_space(..) is reporting that there isn't
  enough free space.

Tested with:	positive/negative cases (see review); make tinderbox

r316953:

Switch back to non-IEC units for 1024 bytes

I was swayed a little too quickly when I saw the wiki page discussing
kB vs KiB. Switch back as none of the code in base openly uses
IEC units via humanize_number(3) (which was my next step), and there's
a large degree of dislike with IEC vs more SI-like units.