librmnetctl: Fix for avc denial in netmgrd

tcontext=u:r:netmgrd:s0 tclass=netlink_socket permissive=0
netmgr calls librmnetctl init where librmnetctl creates socket.
Need to add SOCK_CLOEXEC flag while creating socket.
SOCK_CLOEXEC sets the close-on-exec (FD_CLOEXEC)
flag on the new file descriptor.

Change-Id: I7497737140ae7b0bd8eca27960fec2af209200c7