build: run codespell on almost all files

Ignore only third-party/vendored files (such as license files and files
in m4/).

And ignore more words to fix the following errors:

    $ make codespell
    Running codespell...
    ./README:484: als ==> also
    ./README:646: Shotcut ==> Shortcut
    ./RELNOTES:516: als ==> also
    ./etc/inc/disable-common.inc:506: chage ==> change, charge
    ./etc/apparmor/firejail-default:35: readby ==> read, read by
    ./etc/apparmor/firejail-default:36: readby ==> read, read by
    ./etc/profile-a-l/als.profile:1: als ==> also
    ./etc/profile-a-l/als.profile:5: als ==> also
    make: *** [Makefile:374: codespell] Error 65
    $ codespell --version
    2.2.5

.github/workflows/codespell.yml

@@ -2,13 +2,13 @@ name: Codespell
 
 on:
   push:
-    paths:
-      - 'src/**'
-      - 'test/**'
+    paths-ignore:
+      - 'm4/**'
+      - COPYING
   pull_request:
-    paths:
-      - 'src/**'
-      - 'test/**'
+    paths-ignore:
+      - 'm4/**'
+      - COPYING
 
 permissions:  # added using https://github.com/step-security/secure-workflows
   contents: read

Makefile

@@ -366,12 +366,16 @@ cppcheck: clean
 scan-build: clean
 	scan-build $(MAKE)
 
+# TODO: Old codespell versions (such as v2.1.0 in CI) have issues with
+# contrib/syscalls.sh
 .PHONY: codespell
 codespell:
 	@printf 'Running %s...\n' $@
-	@codespell --ignore-regex "UE|creat|doas|ether|isplay|shotcut" \
+	@codespell --ignore-regex 'UE|als|chage|creat|doas|ether|isplay|readby|[Ss]hotcut' \
 	  -S *.gz,*.o,*.so \
-	  src test
+	  -S COPYING,m4 \
+	  -S ./contrib/syscalls.sh \
+	  .
 
 .PHONY: print-env
 print-env: