...as there are many cases where the code later wants to obtain this part, and
esp. for the string literal variants it is awkward to calculate the length of
the literal again if this is coded with a following copy() call.  Adapt some
code to use this new feature.

(Strictly speaking, the @since tags for the---backwards-compatibly---modified
functions are no longer accurate of course.  Also, clean up some sal_Bool and
SAL_THROWS(()) that are unnecesssary cargo-cult here, and where the clean-up
should have no practical compatibility consequences.)

Change-Id: I43e5c578c8c4b44cb47fd08f170b5c69322ad641