Don't use global variable in libnetutils.

In netd, both CommandListener and NetlinkHandler thread will use
libnetutils' function. Original implementation will use a global
file descriptor in mutiple functions. It will cause problems when
these functions are invoked from different thread.

This change will make NetlinkHandler thread use local file
descriptor instead of global one.

CRs-fixed: 853158
Change-Id: I66458e99e48197e7a56cbb212344423290db12bd