Add a new cnse.fetchscript, which first checks for an absolute path,

then looks in the scripts subdirectory, then in the current directory.
cnse.fetchfile_absolute now checks for an absolute path, then calls
nmap_fetchfile if that fails (and no longer looks in scripts/). Use
cnse.fetchscript when accessing files that should be in the scripts
subdirectory.

nse_fs.h

@@ -1,6 +1,8 @@
 #ifndef NSE_FS
 #define NSE_FS
 
+int fetchscript (lua_State *L);
+
 int fetchfile_absolute (lua_State *L);
 
 int nse_readdir (lua_State *L);