lib: make nslookup, dig, host commands work fine

update posix glue code
 don't override non-socket glibc call for NUSE (getenv, atexit, fseek..)

fcntl partially support (F_DUPFD,F_GETFL,F_SETFL)
 -> introduce struct nuse_socket with reference counter for duplicated fd
pipe(2) handling
non-blokcing connect/recvmsg/sendmsg/accept

poll(2) rework: coexistence of host's fd and nuse's fd
copied a lot from librumphijack/hijack.c

fix epoll_ctl(EPOLL_CTL_DEL) issue of NULL access