parse-util: add parse_tristate() and use it everywhere

We parse tristates all the time, let's add an explicit parser for them.

src/basic/parse-util.h

@@ -12,6 +12,10 @@
 typedef unsigned long loadavg_t;
 
 int parse_boolean(const char *v) _pure_;
+int parse_tristate_full(const char *v, const char *third, int *ret);
+static inline int parse_tristate(const char *v, int *ret) {
+        return parse_tristate_full(v, NULL, ret);
+}
 int parse_pid(const char *s, pid_t* ret_pid);
 int parse_mode(const char *s, mode_t *ret);
 int parse_ifindex(const char *s);