Say "skipped" in scan's warnings for skipped paths (closes #170)

A warning for a path the summary counts as skipped now reads
`scan: skipped PATH (REASON)`, naming the path once. This covers
unreadable directories, failed lstats, failed reads and dropped
operands. Each path of a hard-linked file that cannot be read gets its
own line, since each is counted. Lines for paths left out on purpose
(`--exclude`, default excludes, filesystems that are not persistent)
now read `scan: left out PATH (...)`, so a search for `scan: skipped`
finds only the counted paths.

Model: opus-5-5

README.md

@@ -689,11 +689,10 @@ scan: skipped /srv/a/closed (permission denied)
 scan: skipped /srv/a/pipe (a FIFO)
 ```
 
-The summary counts each path named in such a warning. When a file with several
-hard links cannot be read, each of its paths gets a warning. A line for a path
-left out on purpose starts with `scan: left out` instead, and is not counted. A
-search of the log for `scan: skipped` therefore finds the paths the summary
-counts.
+The summary counts each path named in such a warning. A line for a path left out
+on purpose starts with `scan: left out` instead, and is not counted. A search of
+the log for `scan: skipped` therefore finds the paths the summary counts. When a
+file with several hard links cannot be read, each of its paths gets a warning.
 
 #### Stopping a scan