Add fs_libpath and use within Sys_LoadDll()

Add an additional fs_libpath cvar, which has the same default as
fs_basepath.
Add ability to provide default for fs_libpath by setting
DEFAULT_LIBDIR via Makefile.
Modify Sys_LoadDll() to look into fs_libpath as well.
Rename _Sys_LoadDll() to Sys_LoadDllAbsolute()