stand: liblua: drop default buffer size to 128

Lua allocates LUAL_BUFFERSIZE buffers on the stack for various string
functions (string.format, string.gsub) -- this works out to be somewhat
significant and not necessary, based on how we use string operations.
Dropping it risks having to allocate per call to format/gsub, but this is
not the case for our usage. This simply stops allocating 8K buffers on the
stack when luaL_Buffer is used.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22500