[dev] [dmenu] kpress bugs & patch

Hey all,

I found a few bugs in dmenu tip:
  - inserting text before the end can overflow buffer.
  - inserting text truncates text after cursor.
  - pasting text always appends, and cursor doesn't move.

I know the plan is to remove cursor support (though I'm not sure I
agree), but in the meantime here's a patch. Considering tip can
segfault, perhaps it should be fixed there.

Thanks,
cls