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

README.md

@@ -268,6 +268,8 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
 
 - `mfer gen` / `mfer gen .`
     - recurses under current directory and writes out an `index.mf`
+    - lists regular files only, and skips a named pipe, socket or device, which
+      it names with `-v`
     - records every file's mode as `0000` unless given `--include-permissions`,
       which records each file's permission bits (`0777` at most)
 - `mfer gen /media/drive`
@@ -294,9 +296,10 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
     - rewrites `index.mf` to list the files now under the directory holding it,
       or under the one `--base` names, hashing only the files that are new or
       changed
+    - lists regular files only, as gen does
     - leaves out hidden files unless given `--include-dotfiles`, and symlinks
-      unless given `--follow-symlinks`, which lists each symlink to a file under
-      its own name with the contents of the file it points to
+      unless given `--follow-symlinks`, which lists each symlink to a regular
+      file under its own name with the contents of the file it points to
 - `mfer gen --sign-key key.asc` / `mfer freshen --sign-key key.asc`
     - signs the manifest with the OpenPGP secret key in `key.asc`, armored or
       binary, as `gpg --export-secret-keys` writes it; `MFER_SIGN_KEY` names the