Move freeze() into shared/

Library code should not call freeze(), this is something that should
only be done by "application code", so moving it into shared/ is appropriate.

The fallback to call _exit() is dropped: let's trust that the infinite loop
is infinite.

src/basic/process-util.h

@@ -82,8 +82,6 @@ int pid_from_same_root_fs(pid_t pid);
 
 bool is_main_thread(void);
 
-_noreturn_ void freeze(void);
-
 #ifndef PERSONALITY_INVALID
 /* personality(7) documents that 0xffffffffUL is used for querying the
  * current personality, hence let's use that here as error