C: protect from warning: Redundant argument in printf

we use 1x the long form and for several sections only the short form
C: protect from warning: Redundant argument in sprintf

and uninitialzed $magic warnings.
5.21 is now much stricter with those warnings.
fix t/CORE/comp/retainedlines.t for 5.20
add t/CORE/comp/hints.t: fails sv_magic assert with <=5.14