refactor LinuxSocketFdFactory to more operating system kernels

--HG--
rename : model/linux-socket-fd-factory.cc => model/kernel-socket-fd-factory.cc
rename : model/linux-socket-fd-factory.h => model/kernel-socket-fd-factory.h
rename : model/linux-socket-fd.cc => model/kernel-socket-fd.cc
rename : model/linux-socket-fd.h => model/kernel-socket-fd.h
Bug 1878 - FreeBSD version of network stack for DCE