lib: rename arch/sim to arch/lib

nuse: clean up userspace build and kernelspace build
lib: change the name CONFIG_SIM to CONFIG_LIB

files sim*.[c] indicate to relate to simulator via Direct Code Execution (DCE),
files nuse*.[ch] indicate to relate to Network Stack in Userspace (NUSE).

libsim-linux*.so and libnuse-linux*.so will be generated for both purpose.
code are internally shared.

functions, variables are replaced from 'sim_' to 'lib_'
struct name weren't replaced because ns-3 (DCE) uses this name.

lib: add documentation

This commit addresses the issue #22 and #21.