Opening a named pipe that has no writer blocks, so gen and freshen hung
on a tree holding one, and check hung on a listed path that is a pipe on
disk. gen and freshen now list regular files only (with
--follow-symlinks, what a symlink points to must be one) and name what
they skip with -v. check reports a listed path that is not a regular
file as an error without opening it, and its search for extra files
leaves out pipes, sockets and devices.
Judgement call: the pipe tests build on unix only, where syscall.Mkfifo
exists.
Unverified: sockets and devices are not tested; the tests use pipes.
Model: opus-5-5
