gdb/tui: allow cmd window to change size in tui_layout_split::apply

gdb/tui: allow cmd window to change size in tui_layout_split::apply

When we switch layouts we call the tui_layout_split::apply member
function to reapply the layout, and recalculate all the window sizes.

One special case is the cmd window, which we try to keep at its
existing size.

However, in some cases it is not appropriate to keep the cmd window at
its existing size.  I will describe two such cases here, in one, we
want the cmd window to reduce in size, and in the other, we want the
cmd window to grow in size.

Try these steps in a 80 columns, by 24 lines terminal:

  (gdb) tui enable
  (gdb) layout src
  (gdb) winheight cmd 20
  (gdb) layout split

You should see that the status window is missing from the new layout,
and that the cmd window has been placed over the border of the asm
window.  The 'info win' output is:

  (gdb) info win
  Name       Lines Columns Focus
  src            3      80 (has focus)
  asm            3      80
  status         1      80
  cmd           20      80

Notice that gdb has assigned 27 lines of screen space, even with the
border overlap between the src and asm windows, this is still 2 lines
too many.

The problem here is that after switching layouts, gdb has forced the
cmd window to retain its 20 line height.  Really, we want the cmd
window to reduce in height so that the src and asm windows can occupy
their minimum required space.

This commit allows this (details on how are below).  After this
commit, in the above situation, we now see the status window displayed
correctly, and the 'info win' output is:

  (gdb) info win
  Name       Lines Columns Focus
  src            3      80 (has focus)
  asm            3      80
  status         1      80
  cmd           18      80

The cmd window has been reduced in size by 2 lines so that everything
can fit on the screen.

The second example is one which was discussed in a recent commit,
consider this case (still in the 80 column, 24 line terminal):

  (gdb) tui enable
  (gdb) tui new-layout conly cmd 1
  (gdb) layout conly
  (gdb) info win
  Name       Lines Columns Focus
  cmd            8      80 (has focus)
  (gdb)

This layout only contains a cmd window, which we would expect to
occupy the entire terminal.  But instead, the cmd window only occupies
the first 8 lines, and the rest of the terminal is unused!

The reason is, again, that the cmd window is keeping its previous
size (8 lines).

After this commit things are slightly different, the 'info win' output
is now:

  (gdb) info win
  Name       Lines Columns Focus
  cmd           20      80 (has focus)

Which is a little better, but why only 20 lines?  Turns out there's
yet another bug hitting this case.  That bug will be addressed in a
later commit, so, for now, we're accepting the 20 lines.

What this commit does is modify the phase of tui_layout_split::apply
that handles any left over space.  Usually, in "Step 2", each
sub-layout has a size calculated.  As the size is an integer, then,
when all sizes are calculated we may have some space left over.

This extra space is then distributed between all the windows fairly
until all the space is used up.

When we consider windows minimum size, or fixed size windows, then it
is possible that we might try to use more space than is available,
this was our first example above.  The same code that added extra
space to the windows, can also be used to reclaim space (in the over
allocation case) to allow all windows to fit.

The problem then is the cmd window, which we often force to a fixed
size.  Inside the loop that handles the allocation of excess space, if
we find that we have tried every window, and still have space either
left to give, or we need to claim back more space, then, if the cmd
window was changed to a fixed size, we can change the cmd window back
to a non-fixed-size window, and proceed to either give, or take space
from the cmd window as needed.