Read only regular files in gen, freshen and check (closes #189)

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

internal/cli/freshen.go

@@ -192,6 +192,13 @@ func (s *freshenScanner) walk(path string, info fs.FileInfo, walkErr error) erro
 		info = realInfo
 	}
 
+	// Opening a named pipe can block, and reading a device may never end.
+	if !info.Mode().IsRegular() {
+		log.Verbosef("skipping %s: not a regular file", relPath)
+
+		return nil
+	}
+
 	for _, excluded := range s.excluded {
 		if os.SameFile(info, excluded) {
 			return nil