lua: fix use of renamed function

Apparently this was overlooked when get_timer was renamed to get_time.
client API: fix timeout handling

(Again.)

Fixed Lua timers as well.
lua: fix format string in luaL_error()
lua: fix add_key_binding()

add_key_binding() didn't work, because it passed a flag that was
renamed. add_forced_key_binding() worked, but did the wrong thing.