Small fix with sadc's option -S

sadc was unable to read a list of comma separated values following
option -S. This is now fixed.

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

Add a new entry to sysstat FAQ to explain how to collect only specific
activities with sadc.

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