tree-wide: make use of getpid_cached() wherever we can

This moves pretty much all uses of getpid() over to getpid_raw(). I
didn't specifically check whether the optimization is worth it for each
replacement, but in order to keep things simple and systematic I
switched over everything at once.

src/basic/util.c

@@ -219,7 +219,7 @@ int fork_agent(pid_t *pid, const int except[], unsigned n_except, const char *pa
         /* Spawns a temporary TTY agent, making sure it goes away when
          * we go away */
 
-        parent_pid = getpid();
+        parent_pid = getpid_cached();
 
         /* First we temporarily block all signals, so that the new
          * child has them blocked initially. This way, we can be sure