Leave caches out of the walk by default (closes #181)

The default excludes now cover caches that programs and package
managers fill again on their own. On Linux: /var/cache and, in /root
and in each home in /home, .cache, .npm, .cargo/registry and
go/pkg/mod. On macOS: /private/var/folders, /Library/Caches and, in
/private/var/root and in each home in /Users, Library/Caches plus the
same four. README and scan --help list them; --no-default-excludes
still turns them all off.

Model: opus-5-5

README.md

@@ -128,8 +128,8 @@ walk"). As the file `/etc/cron.d/sfdupes`:
     - one line naming the database;
     - progress lines, as described under "Progress";
     - a warning for each path it skips;
-    - a line for each system or temporary directory the walk leaves out (see
-      "Rules for the walk");
+    - a line for each system, temporary or cache directory the walk leaves out
+      (see "Rules for the walk");
     - a line for each path left out because its filesystem is one the kernel,
       the core of the operating system, makes up or keeps in memory, such as
       `tmpfs`;
@@ -300,8 +300,9 @@ The reference below is also the program's specification.
   of directories on other filesystems, as the same flag does for `du` and
   `rsync`. By default the walk also enters directories on other filesystems.
 - `--no-default-excludes` (`scan`, `dwim`) lets the walk enter the default
-  excludes. Those are the system and temporary directories the walk otherwise
-  leaves out, such as `/var/tmp` on Linux (see "Rules for the walk").
+  excludes. Those are the system, temporary and cache directories the walk
+  otherwise leaves out, such as `/var/tmp` and `/var/cache` on Linux (see "Rules
+  for the walk").
 - `--exclude PATH` (`scan`, `dwim`) keeps the walk out of `PATH` and everything
   below it. `PATH` may be a directory or a file. The flag may be given more than
   once. The walk knows `PATH` by its device ID and inode number, as it knows the
@@ -571,12 +572,17 @@ walk practical on large or busy storage pools.
     `--no-default-excludes` does not change this rule.
 
 - Below an operand, the walk does not enter a default exclude. On Linux, the
-  default excludes hold temporary files and the kernel's view of the running
-  system. They depend on the system, and `sfdupes scan --help` lists those of
-  the system it runs on. On Linux they are:
+  default excludes hold the kernel's view of the running system, temporary
+  files, and caches. A cache here is data that programs and package managers
+  download or build again on their own. The default excludes depend on the
+  system, and `sfdupes scan --help` lists those of the system it runs on. In the
+  lists, `*` stands for any one directory name, so `/home/*/.cache` matches
+  `/home/alice/.cache`. On Linux they are:
 
     ```
-    /proc, /sys, /dev, /run, /tmp, /var/tmp
+    /proc, /sys, /dev, /run, /tmp, /var/tmp, /var/cache
+    /root/.cache, /root/.npm, /root/.cargo/registry, /root/go/pkg/mod
+    /home/*/.cache, /home/*/.npm, /home/*/.cargo/registry, /home/*/go/pkg/mod
     ```
 
     On macOS they are:
@@ -584,13 +590,21 @@ walk practical on large or busy storage pools.
     ```
     /dev, /private/tmp, /private/var/tmp, /private/var/vm
     /System/Volumes/VM, /System/Volumes/Data
+    /private/var/folders, /Library/Caches
+    /private/var/root/Library/Caches, /private/var/root/.cache
+    /private/var/root/.npm, /private/var/root/.cargo/registry
+    /private/var/root/go/pkg/mod
+    /Users/*/Library/Caches, /Users/*/.cache, /Users/*/.npm
+    /Users/*/.cargo/registry, /Users/*/go/pkg/mod
     ```
 
-    A walk of `/` on macOS reaches everything in `/System/Volumes/Data` already,
-    through firmlinks. Firmlinks are the links macOS uses to show the data
-    volume inside the system volume. Walking `/System/Volumes/Data` as well
-    would record each of those files twice. Any other system has no default
-    excludes.
+    `/private/var/folders` holds each user's temporary files and caches on
+    macOS. Root's home on macOS is `/var/root`, which the walk reaches as
+    `/private/var/root`. A walk of `/` on macOS reaches everything in
+    `/System/Volumes/Data` already, through firmlinks. Firmlinks are the links
+    macOS uses to show the data volume inside the system volume. Walking
+    `/System/Volumes/Data` as well would record each of those files twice. Any
+    other system has no default excludes.
 
     An operand at or inside a default exclude is walked, so
     `sfdupes scan /tmp/x` works when `/tmp/x` is on a persistent filesystem.