lib/repo: Add min-free-space-percent option, default 3%

lib/repo: Add min-free-space-percent option, default 3%

For ostree-as-host, we're the superuser, so we'll blow past
any reserved free space by default.  While deltas have size
metadata, if one happens to do a loose fetch, we can fill
up the disk.

Another case is flatpak: the system helper has similar concerns
here as ostree-as-host, and for `flatpak --user`, we also
want to be nice and avoid filling up the user's quota.

Closes: https://github.com/ostreedev/ostree/issues/962
Closes: #987
Approved by: jlebon