Make linux_nat_detach/thread_db_detach use the inferior parameter

This patch makes these two functions actually use the inferior parameter
added by the previous patch, instead of reading inferior_ptid.  I chose
these two, because they are the one actually used when I detach on my
GNU/Linux system, so they were easy to test.

I took the opportunity to pass the inferior being detached to
inf_ptrace_detach_success, so it could use it too.  From there, it made
sense to add an overload of detach_inferior that takes the inferior
directly rather than the pid, to avoid having to pass inf->pid only for
the callee to look up the inferior structure by pid.

gdb/ChangeLog:

	* inf-ptrace.c (inf_ptrace_detach): Adjust call to
	inf_ptrace_detach_success.
	(inf_ptrace_detach_success): Add inferior parameter, use it
	instead of inferior_ptid, pass it to detach_inferior.
	* inf-ptrace.h (inf_ptrace_detach_success): Add inferior
	parameter.
	* inferior.c (detach_inferior): Add overload that takes an
	inferior object.
	* inferior.h (detach_inferior): Likewise.
	* linux-nat.c (linux_nat_detach): Use the inf parameter, don't
	use inferior_ptid, adjust call to inf_ptrace_detach_success.
	* linux-thread-db.c (thread_db_detach): Use inf parameter.

gdb/linux-thread-db.c

@@ -1094,7 +1094,7 @@ thread_db_detach (struct target_ops *ops, inferior *inf, int from_tty)
 {
   struct target_ops *target_beneath = find_target_beneath (ops);
 
-  delete_thread_db_info (ptid_get_pid (inferior_ptid));
+  delete_thread_db_info (inf->pid);
 
   target_beneath->to_detach (target_beneath, inf, from_tty);