process-util: add new helper call for adjusting the OOM score

And let's make use of it in execute.c

src/basic/process-util.c

@@ -1445,6 +1445,15 @@ int fork_agent(const char *name, const int except[], size_t n_except, pid_t *ret
         _exit(EXIT_FAILURE);
 }
 
+int set_oom_score_adjust(int value) {
+        char t[DECIMAL_STR_MAX(int)];
+
+        sprintf(t, "%i", value);
+
+        return write_string_file("/proc/self/oom_score_adj", t,
+                                 WRITE_STRING_FILE_VERIFY_ON_FAILURE|WRITE_STRING_FILE_DISABLE_BUFFER);
+}
+
 static const char *const ioprio_class_table[] = {
         [IOPRIO_CLASS_NONE] = "none",
         [IOPRIO_CLASS_RT] = "realtime",