nfct: use cth for polling

This patch uses cth for polling instead of pgh. pgh is now for purging
private.
nfct: initialize at constructor_nfct as possible
nfct: construct by passing handler
nfct: introduce counter mode

This patch introduce counter mode, which aim for nfct-daemon.c in
libmnl working in ulogd.
sprint: fix realloc length

and make valgrind happy by memset it.
nfct: introduce gauge counter

by reseting counter every dump.