Fix PR gdb/19828: gdb -p <process from a container>: internal error

Fix PR gdb/19828: gdb -p <process from a container>: internal error

When GDB attaches to a process, it looks at the /proc/PID/task/ dir
for all clone threads of that process, and attaches to each of them.

Usually, if there is more than one clone thread, it means the program
is multi threaded and linked with pthreads.  Thus when GDB soon after
attaching finds and loads a libthread_db matching the process, it'll
add a thread to the thread list for each of the initially found
lower-level LWPs.

If, however, GDB fails to find/load a matching libthread_db, nothing
is adding the LWPs to the thread list.  And because of that, "detach"
hits an internal error:

  (gdb) PASS: gdb.threads/clone-attach-detach.exp: fg attach 1: attach
  info threads
    Id   Target Id         Frame
  * 1    LWP 6891 "clone-attach-de" 0x00007f87e5fd0790 in __nanosleep_nocancel () at ../sysdeps/unix/syscall-template.S:84
  (gdb) FAIL: gdb.threads/clone-attach-detach.exp: fg attach 1: info threads shows two LWPs
  detach
  .../src/gdb/thread.c:1010: internal-error: is_executing: Assertion `tp' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n)
  FAIL: gdb.threads/clone-attach-detach.exp: fg attach 1: detach (GDB internal error)

From here:

  ...
  #8  0x00000000007ba7cc in internal_error (file=0x98ea68 ".../src/gdb/thread.c", line=1010, fmt=0x98ea30 "%s: Assertion `%s' failed.")
      at .../src/gdb/common/errors.c:55
  #9  0x000000000064bb83 in is_executing (ptid=...) at .../src/gdb/thread.c:1010
  #10 0x00000000004c23bb in get_pending_status (lp=0x12c5cc0, status=0x7fffffffdc0c) at .../src/gdb/linux-nat.c:1235
  #11 0x00000000004c2738 in detach_callback (lp=0x12c5cc0, data=0x0) at .../src/gdb/linux-nat.c:1317
  #12 0x00000000004c1a2a in iterate_over_lwps (filter=..., callback=0x4c2599 <detach_callback>, data=0x0) at .../src/gdb/linux-nat.c:899
  #13 0x00000000004c295c in linux_nat_detach (ops=0xe7bd30, args=0x0, from_tty=1) at .../src/gdb/linux-nat.c:1358
  #14 0x000000000068284d in delegate_detach (self=0xe7bd30, arg1=0x0, arg2=1) at .../src/gdb/target-delegates.c:34
  #15 0x0000000000694141 in target_detach (args=0x0, from_tty=1) at .../src/gdb/target.c:2241
  #16 0x0000000000630582 in detach_command (args=0x0, from_tty=1) at .../src/gdb/infcmd.c:2975
  ...

Tested on x86-64 Fedora 23.  Also confirmed the test passes against
gdbserver with "maint set target-non-stop".

gdb/ChangeLog:
2016-05-24  Pedro Alves  <palves@redhat.com>

	PR gdb/19828
	* linux-nat.c (attach_proc_task_lwp_callback): Mark the lwp
	resumed, and add the thread to GDB's thread list.

testsuite/ChangeLog:
2016-05-24  Pedro Alves  <palves@redhat.com>

	PR gdb/19828
	* gdb.threads/clone-attach-detach.c: New file.
	* gdb.threads/clone-attach-detach.exp: New file.