iostat: Major code refactoring

iostat: Major code refactoring

This patch adds the following features:
* Structures for devices statistics are now dynamically allocated. It is
no longer possible to miss a device if many of them are added into the
system while iostat is running.
* Check for devices which are removed then inserted again into the
system. iostat will no longer display abormally high numbers in these
cases.
* Better handle devices entered on the command line, which may be
partitions and/or whole devices.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>