server: add polling wrapper for unix poll and epoll

Knot.files

configure.ac

src/knot/Makefile.inc

src/knot/common/apoll.h

src/knot/common/epoll_ctx.c

src/knot/common/epoll_ctx.h

src/knot/common/fdset.c

src/knot/common/fdset.h

src/knot/server/tcp-handler.c

src/knot/server/udp-handler.c

src/utils/knotd/main.c