lib: Add explicit return statements

Add explicit return statements to subs.  Subs without a "well defined"
return value or where the return value is not used, a simple "return;"
was added.

Exceptions include {,l,r}strip, which explicitly behave differently in
void context (and thus a return for that does not really matter).

Signed-off-by: Niels Thykier <niels@thykier.net>