Unify use of the schem(e/a) word

Knot.files

src/Makefile.am

src/knot/conf/base.c

src/knot/conf/base.h

src/knot/conf/conf.h

src/knot/conf/confdb.c

src/knot/conf/confdb.h

src/knot/conf/confio.c

src/knot/conf/migration.c

src/knot/conf/migration.h

src/knot/conf/module.c

src/knot/conf/schema.c

src/knot/conf/schema.h

src/knot/conf/tools.c

src/knot/conf/tools.h

src/knot/dnssec/kasp/keystore.c

src/knot/include/module.h

src/knot/modules/dnsproxy/dnsproxy.c

src/knot/modules/rosedb/rosedb.c

src/knot/modules/rrl/rrl.c