luci-mod-status: use available and cached memory for progress bars

Fixes: #1148

Uses available and cached memory from /proc/meminfo via procd, and carefully
handles the cases where they are nil or zero, so as to not require specific
linux or procd versions to function

MemAvailable is a better estimate than free + buffered/cached, see:
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0

Also adds a new progressbar that shows cached memory if not nil

Signed-off-by: Zachary Cook <zachcook1991@gmail.com>