Resolve check and freshen paths against the manifest's directory (closes #177)

Without --base, check and freshen now look for a manifest's files in the
directory that holds the manifest, the file named or the one found in a
directory argument, instead of the current directory. check of a manifest
given by URL still uses the current directory, and a --base given on the
command line, the current directory included, still overrides. The --base
help text of both commands and the README's Tool Examples state the
default; the README gains a freshen entry to do so.

Model: opus-5-5

README.md

@@ -273,9 +273,17 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
     - verifies checksums of all files in manifest, displaying error and exiting
       nonzero if any files are missing or corrupted, or have permission bits
       other than the mode the manifest records, unless that is `0000`
+    - looks for those files under the base directory: the one `--base` names, or
+      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
+- `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
+      changed
 - `mfer fetch https://example.com/stuff/`
     - fetches `/stuff/index.mf` and downloads all files listed in manifest into
       the current directory, or the one given with `--dest`, and assures