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

@@ -563,7 +563,7 @@ walk practical on large or busy storage pools.
     `scan` prints a line on stderr naming the path and its filesystem type:
 
     ```
-    scan: skipped /dev/shm (tmpfs, not a persistent filesystem)
+    scan: left out /dev/shm (tmpfs, not a persistent filesystem)
     ```
 
     The path is not counted as skipped. The records below it are kept, as
@@ -573,7 +573,7 @@ walk practical on large or busy storage pools.
   For each one it reaches, `scan` prints a line on stderr naming it:
 
     ```
-    scan: skipped /srv/cache (--exclude)
+    scan: left out /srv/cache (--exclude)
     ```
 
     The path is not counted as skipped. The scan keeps the records at and below
@@ -622,7 +622,7 @@ walk practical on large or busy storage pools.
     not enter, `scan` prints a line on stderr naming it:
 
     ```
-    scan: skipped /var/tmp (default exclude)
+    scan: left out /var/tmp (default exclude)
     ```
 
     The directory is not counted as skipped. `scan` prints the filesystem line
@@ -682,6 +682,18 @@ scan: 123400 files seen (1200 added, 34 updated, 56 removed, 122166 unchanged),
 
 `removed` counts deleted records, which are not part of the files seen.
 
+The warning for a skipped path names the path once and gives the reason:
+
+```
+scan: skipped /srv/a/closed (permission denied)
+scan: skipped /srv/a/pipe (a FIFO)
+```
+
+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
 
 `scan` stops cleanly on `SIGINT`, the signal Ctrl-C sends, or `SIGTERM`, the