Make sysstat init script source the functions library

Make sysstat init script source the functions library.
This is needed to conform to the LSB Core Specifications and
severeal other tools depend on it.

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