Report only symlinks to regular files as extra files in check (closes #192)

gen never lists a symlink to a directory or a broken one, and lists a
symlink to a regular file only with --follow-symlinks. check's search for
extra files reported every symlink, so on a tree holding a directory
symlink no manifest gen writes could pass check --no-extra-files. The
search now resolves a symlink and reports an entry only if it is a
regular file, which also covers the pipes, sockets and devices it already
left out. The README's check entry says what is reported.

Judgement call: one regular-file check replaces the separate test for
pipes, sockets and devices, so a file of unknown type is not reported
either; gen does not list one.

Model: opus-5-5

README.md

@@ -289,9 +289,12 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
       else the directory holding the manifest, or the current directory for a
       manifest given by URL. So `mfer check /media/drive` checks a drive against
       the `index.mf` at its root, from any directory
-    - warns about each file under the base directory that the manifest does not
-      list, hidden files included; with `--no-extra-files` each one is a failure
-      instead
+    - warns about each regular file under the base directory that the manifest
+      does not list, hidden files included, and each symlink there to a regular
+      file, which gen lists only with `--follow-symlinks`; with
+      `--no-extra-files` each one is a failure instead. A named pipe, socket or
+      device, a symlink to a directory and a broken symlink are never reported,
+      since gen cannot list them
 - `mfer freshen` / `mfer freshen .`
     - 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