lib: Enable consecutive frrscript_call

lib: Enable consecutive frrscript_call

Previous:
 - frrscript_load: push Lua function onto stack
 - frrscript_call: calls Lua function

Now:
 - frrscript_load: checks Lua function
 - frrscript_call: pushes and calls Lua function (first clear the stack)

So now we just need one frrscript_load for consecutive frrscript_call.
frrscript_call does not recompile or reload the script file, it just
keys it from the global Lua table (where it should already be).

Signed-off-by: Donald Lee <dlqs@gmx.com>