Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Use '%zu' and '%zd' as appropriate for size_t / ssize_t.

Small regression test tool to generate two forms of ENOSPC on a file
system: out of blocks, and out of inodes. Useful for exercising the
uprintf(9) calls in UFS/ext2fs in order to detect races.