Introduce add_ms_to_timeb_diff().

Most calls to add_ms_to_timeb() do not need the return value, so this
introduces add_ms_to_timeb_diff() which lets us remove cs_ftime() call
from add_ms_to_timeb().

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@9376 4b0bc96b-bc66-0410-9d44-ebda105a78c1