lua: restore return value of require('uci') to module table.

lua: restore return value of require('uci') to module table.

Before commit "b121dd lua: add lua 5.2 compatibility changes", the
module table was filled by luaL_register() which would leave the table
at the top of the stack.  But in current code the module table will be
popped by lua_setglobal() causing the return value defaulting to boolean
value true.

Also fix the return value of luaopen_uci() to 1 to indicate the number
of return value on stack.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>