tree-wide: add a helper to parse boolean optarg

This nicely covers the case when optarg is optional. The same parser can be
used when the option string passed to getopt_long() requires a parameter and
when it doesn't.

The error messages are made consistent.
Also fixes a log error c&p in --crash-reboot message.

src/shared/parse-argument.c

@@ -10,6 +10,25 @@
 
 /* All functions in this file emit warnigs. */
 
+int parse_boolean_argument(const char *optname, const char *s, bool *ret) {
+        int r;
+
+        /* Returns the result through *ret and the return value. */
+
+        if (s) {
+                r = parse_boolean(s);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to parse boolean argument to %s: %s.", optname, s);
+
+                *ret = r;
+                return r;
+        } else {
+                /* s may be NULL. This is controlled by getopt_long() parameters. */
+                *ret = true;
+                return true;
+        }
+}
+
 int parse_json_argument(const char *s, JsonFormatFlags *ret) {
         assert(s);
         assert(ret);