Add PageTables information

[anayrat]

Hi Gilles,

This PR add kernel PageTables information. It is the amount of memory used by process to address shared memory :

PageTables: Amount of memory dedicated to the lowest level of page tables. This can increase to a high value if a lot of processes are attached to the same shared memory segment.

https://access.redhat.com/solutions/406773

Try a pgbench with -c 4 and -c 800. You'll see ;)

Cheers!