Debug into glibc with source code display in gdb on Debian

[Zhaoming Luo]

Hi,

I'm trying to track the curl failed tests by debugging select() function
(or system call?) defined in glibc using gdb. When I executed `b _hurd_select`
to set a break point and run the program. The gdb stopped at the breakpoint
but it gives me the following info:

```
Thread 2 hit Breakpoint 1, _hurd_select (nfds=7, pollfds=0x0, 
readfds=0x103cc20, writefds=0x103cc40, exceptfds=0x103cc60,
    timeout=0x103cba4, sigmask=0x0) at ./hurd/hurdselect.c:51
51      ./hurd/hurdselect.c: No such file or directory.
```

I expected it will show me the source code of hurdselect.c.

There is a simpler program to reproduce: write a hello world program and
compile with -g option, step into printf in gdb. Here is the output
info:

```
Thread 4 hit Breakpoint 1, main () at main.c:6
6           printf ("Hello World\n");
(gdb) s
0x010ff074 in __GI__IO_puts (str=0x1518 "Hello World") at ./libio/ioputs.c:33
33      ./libio/ioputs.c: No such file or directory.
```

Using `list` or `l` command it shows only the following:

```
(gdb) l
30      in ./libio/ioputs.c
```

On my Arch GNU/Linux it automatically downloaded the source code and
give the following (the last two lines are what I expected):

```
Breakpoint 1, main () at main.c:6
6           printf ("Hello\n");
(gdb) s
0x00007ffff7e1fa78 in __GI__IO_puts (str=0x555555556004 "Hello") at ioputs.c:35
35        size_t len = strlen (str);
```

The libc0.3-dbg package is installed on Debian Hurd:
```
libc0.3-dbg/unstable,now 2.40-6 hurd-i386 [installed]
  GNU C Library: detached debugging symbols
```

I strongly suspects I missed a package but I cannot find which one as I
have the same issue on a Debian GNU/Linux virtual machine :/.

Any help is appreciated :).

Best,
Zhaoming