pidref: add PIDREF_MAKE_FROM_PID()

This helper truns a pid_t into a PidRef. It's different from
pidref_set_pid() in being "passive", i.e. it does not attempt to acquire
a pidfd for the pid.

This is useful when using the PidRef as a lookup key that shall also
work after a process is already dead, and hence no conversion to a pidfd
is possible anymore.

src/basic/pidref.c

@@ -68,11 +68,7 @@ int pidref_set_pidfd(PidRef *pidref, int fd) {
                 if (r < 0)
                         return r;
 
-                *pidref = (PidRef) {
-                        .fd = -EBADF,
-                        .pid = pid,
-                };
-
+                *pidref = PIDREF_MAKE_FROM_PID(pid);
                 return 0;
         }