add regression tests

- check perl syntax
- run mandoc lint on man page
- check README.md is up to date
- check .Dd date in man page is accurate

Makefile

@@ -14,4 +14,26 @@ realinstall:
 README.md: sysclean.8
 	mandoc -T markdown sysclean.8 >$@
 
+regress: run-regress-perl-syntax \
+	run-regress-man-lint \
+	run-regress-man-readme \
+	run-regress-man-date
+
+run-regress-perl-syntax:
+	perl -c sysclean.pl
+
+run-regress-man-lint:
+	mandoc -T lint sysclean.8
+
+run-regress-man-readme:
+	mandoc -T markdown sysclean.8 | diff -q README.md -
+
+run-regress-man-date:
+	if [ -d .git ]; then \
+		grep -qF -- \
+			"$$(date -r $$(git log -1 --format="%ct" sysclean.8) \
+				+'.Dd %B %d, %Y')" \
+			sysclean.8 ; \
+	fi
+
 .include <bsd.prog.mk>