Add WITNESS_WARN() to getenv() to explicitly note that the function may

sleep. This helps to expose bugs when the requested environment variable
doesn't exist.

sys/kern/kern_environment.c

@@ -310,6 +310,7 @@ getenv(const char *name)
 	int len;
 
 	if (dynamic_kenv) {
+		WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL, "getenv");
 		mtx_lock(&kenv_lock);
 		cp = _getenv_dynamic(name, NULL);
 		if (cp != NULL) {