raw-clone: beef up raw_clone() wrapper a bit

First of all, let's return pid_t, which appears to be the correct type
given that we return PIDs, and it#s what fork() uses too.

Most importantly though, flush out our PID cache, so that the call
becomes compatible with our getpid_cached() logic.

src/basic/process-util.h

@@ -147,6 +147,7 @@ static inline int sched_policy_to_string_alloc_with_check(int n, char **s) {
 int ioprio_parse_priority(const char *s, int *ret);
 
 pid_t getpid_cached(void);
+void reset_cached_pid(void);
 
 int must_be_root(void);