gdb: don't share aspace/pspace on fork with "detach-on-fork on" and "follow-fork...

gdb: don't share aspace/pspace on fork with "detach-on-fork on" and "follow-fork-mode child"

We found that when handling forks, two inferiors can unexpectedly share
their program space and address space.  To reproduce:

 1. Using a test program that forks...
 2. "set follow-fork-mode child"
 3. "set detach-on-fork on" (the default)
 4. run to a breakpoint somewhere after the fork

Step 4 should have created a new inferior:

    (gdb) info inferiors
      Num  Description       Connection           Executable
      1    <null>                                 /home/smarchi/build/wt/amd/gdb/fork
    * 2    process 251425    1 (native)           /home/smarchi/build/wt/amd/gdb/fork

By inspecting the state of GDB, we can see that the two inferiors now
share one program space and one address space:

Inferior 1:

    (top-gdb) p inferior_list.m_front.num
    $2 = 1
    (top-gdb) p inferior_list.m_front.aspace
    $3 = (struct address_space *) 0x5595e2520400
    (top-gdb) p inferior_list.m_front.pspace
    $4 = (struct program_space *) 0x5595e2520440

Inferior 2:

    (top-gdb) p inferior_list.m_front.next.num
    $5 = 2
    (top-gdb) p inferior_list.m_front.next.aspace
    $6 = (struct address_space *) 0x5595e2520400
    (top-gdb) p inferior_list.m_front.next.pspace
    $7 = (struct program_space *) 0x5595e2520440

You can then run inferior 1 again and the two inferiors will still
erroneously share their spaces, but already at this point this is wrong.

The cause of the bad {a,p}space sharing is in follow_fork_inferior.
When following the child and detaching from the parent, we just re-use
the parent's spaces, rather than cloning them.  When we switch back to
inferior 1 and run again, we find ourselves with two unrelated inferiors
sharing spaces.

Fix that by creating new spaces for the parent after having moved them
to the child.  My initial implementation created new spaces for the
child instead.  Doing this breaks doing "next" over fork().  When "next"
start, we record the symtab of the starting location.  When the program
stops, we compare that symtab with the symtab the program has stopped
at.  If the symtab or the line number has changed, we conclude the
"next" is done.  If we create a new program space for the child and copy
the parent's program space to it with clone_program_space, it creates
new symtabs for the child as well.  When the child stop, but still on
the fork() line, GDB thinks the "next" is done because the symtab
pointers no longer match.  In reality they are two symtab instances that
represent the same file.  But moving the spaces to the child and
creating new spaces for the parent, we avoid this problem.

Note that the problem described above happens today with "detach-on-fork
off" and "follow-fork-mode child", because we create new spaces for the
child.  This will have to be addressed later.

Test-wise, improve gdb.base/foll-fork.exp to set a breakpoint that is
expected to have a location in each inferiors.  Without the fix, when
the two inferiors erroneously share a program space, GDB reports a
single location.

Change-Id: Ifea76e14f87b9f7321fc3a766217061190e71c6e