Include mbcs_pool in all mbcs values

In R16B02 an option to enable a mbc pool was added. In order to
get the total size of current mbcs we have to include the pool in
all our calculations with mbcs.

This commit also does a bit of refactoring that makes the code
more modular when fetching info from within the allocator statistics.

I've tested this with Erlang/OTP R15B03, R16B03 and 17.0. 

Note that this still not the complete total of all memory requested by the Erlang VM. There is also a the mseg cache which can contain up to 30 segments of varying size. In 17.0 it is not possible to get size of the segments cached (only the number of segments), so it is not currently possible to get the complete picture. I plan to add stats for getting the cached segment sizes in Erlang/OTP 17.1 and will hopefully remember to add that sum to recon_alloc.

This is bug fix.