basic: split parsing of the OOM score adjust value into its own function in parse-util.c

And port config_parse_exec_oom_score_adjust() over to use it.

While we are at it, let's also fix config_parse_exec_oom_score_adjust()
to accept an empty string for turning off OOM score adjustments set
earlier.

src/basic/parse-util.c

@@ -676,3 +676,20 @@ int parse_dev(const char *s, dev_t *ret) {
         *ret = d;
         return 0;
 }
+
+int parse_oom_score_adjust(const char *s, int *ret) {
+        int r, v;
+
+        assert(s);
+        assert(ret);
+
+        r = safe_atoi(s, &v);
+        if (r < 0)
+                return r;
+
+        if (v < OOM_SCORE_ADJ_MIN || v > OOM_SCORE_ADJ_MAX)
+                return -ERANGE;
+
+        *ret = v;
+        return 0;
+}

src/basic/parse-util.h

@@ -118,3 +118,5 @@ int parse_percent(const char *p);
 int parse_nice(const char *p, int *ret);
 
 int parse_ip_port(const char *s, uint16_t *ret);
+
+int parse_oom_score_adjust(const char *s, int *ret);