check_mem - get the MemAvailable counter (kernel 3.14+)

/proc/meminfo provides an estimated available memory counter,
starting by kernel 3.14.
MemAvailable gives an estimation of how much memory is available
for starting new applications, without swapping.

This commit adds a new switch -a/--available for switching to this
new counter instead of the default one (the memory used counter).

See kernel commit:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773

Signed-off-by: Davide Madrisan <davide.madrisan@gmail.com>