Make sadf -H display last CPU count in file

sadf -H now displays the last CPU count recorded in the data file. This
is the value of the last_cpu_nr field saved in file's header, giving the
number of processors the machine had when the last sample of statistics
was appended to the file.

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

Update FAQ and tell that data can now be appended to a datafile even if
the number of processors of the machine has changed.

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