makefs: Handle special file types when creating a zpool

Previously, anything other than a regular file, directory or symlink
would cause makefs to exit with an assertion failure. Make it a bit
more resilient to user error: print a warning and skip the file. Add a
regression test wherein we create an image from a devfs mount.

PR: 283583
MFC after: 2 weeks

(cherry picked from commit ce878284318e71217d8d8f43f7d590b6c338d3aa)