free: add 'available' memory if provided by the kernel

free: add 'available' memory if provided by the kernel

Show estimated available memory if this is provided by the
kernel. See [1] for the full story.

 [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773

function                                             old     new   delta
free_main                                            537     633     +96
parse_cached_kb                                       85       -     -85
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 1/0 up/down: 96/-85)             Total: 11 bytes

Signed-off-by: Guillermo Rodriguez <guille.rodriguez@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>