knotc: add blocking mode for zone event triggers

doc/man/knotc.8in

doc/man_knotc.rst

src/knot/ctl/commands.c

src/knot/ctl/commands.h

src/utils/knotc/commands.c

src/utils/knotc/commands.h

src/utils/knotc/main.c

src/utils/knotc/process.c

src/utils/knotc/process.h