Allocate extra inodes in makefs when leaving free space in UFS images.

By default, makefs(8) has very few spare inodes in its output images,
which is fine for static filesystems, but not so great for VM images
where many more files will be added. Make makefs(8) use the same
default settings as newfs(8) when creating images with free space --
there isn't much point to leaving free space on the image if you
can't put files there. If no free space is requested, use current
behavior of a minimal number of available inodes.

Reviewed by: manu
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D29492

(cherry picked from commit afb6a168f8ee08ac74769464726c396fbef83d0b)