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.h

@@ -173,6 +173,8 @@ static inline int safe_fork(const char *name, ForkFlags flags, pid_t *ret_pid) {
 
 int fork_agent(const char *name, const int except[], size_t n_except, pid_t *pid, const char *path, ...);
 
+int set_oom_score_adjust(int value);
+
 #if SIZEOF_PID_T == 4
 /* The highest possibly (theoretic) pid_t value on this architecture. */
 #define PID_T_MAX ((pid_t) INT32_MAX)