contrib: move string from internal mem

src/Makefile.am

src/contrib/string.c

src/contrib/string.h

src/knot/conf/base.c

src/knot/conf/conf.c

src/knot/conf/confio.c

src/knot/ctl/estimator.c

src/knot/ctl/knotc_main.c

src/knot/ctl/remote.c

src/knot/dnssec/zone-events.c

src/knot/modules/online_sign/module.c

src/knot/modules/rosedb_tool.c

src/knot/zone/timers.c

src/knot/zone/zone.c

src/knot/zone/zonefile.c

src/libknot/binary.c

src/libknot/rrtype/nsec3param.c

src/libknot/tsig.c

src/utils/knot1to2/main.c

src/utils/knsupdate/knsupdate_exec.c