Use a lightweight static library to compile some sysstat commands

Create two versions of the "librdstats.a" static library: one having all
the functions to read stats and which will be used by sadc, the other
having only a minimal subset of functions used by other commands like
iostat or pidstat.
The result is a much smaller binary file for iostat (size is 29% smaller
than before), pidstat (-27%), mpstat (-40%), nfsiostat (-43%) and
cifsiostat (-43%).