Lua fix strip_extension and create_numbered_filename

Both of these functions modified the string in the lua stack

per lua manual:
'When a C function receives a string argument from Lua,
there are only two rules that it must observe: Not to pop the string
from the stack while accessing it and never to modify the string'

strip_extension will still work with old parameters
and is thus backwards compatible
strip_extension("filename")

create_numbered_filename has changed slightly and IS NOT backwards compatible

create_numbered_filename(path, prefix, suffix, [number])
(number defaults to -1)

Change-Id: I34cf7e2f6f691f33d5ac2b2e995855a171fb99b3

apps/plugins/lua/rocklib_aux.pl

@@ -65,6 +65,8 @@ my @forbidden_functions = ('^open$',
                            '^fdprintf$',
                            '^read_line$',
                            '^[a-z]+dir$',
+                           '^strip_extension$',
+                           '^create_numbered_filename$',
                            '^s?+rand$',
                            '^strl?+cpy$',
                            '^strl?+cat$',