Problem:
Border width calculations were scattered with repeated
`*p_pumborder != NUL ? 2 : 0` patterns. The "none" value was not
consistently checked, causing borders to appear when pumborder="none".
When "shadow" the info floating window have an extra cell of spacing.
Solution:
Add `pum_border_width()` helper that returns 0 when pumborder is unset
or "none" (opt_winborder_values[7]), returns 1 when pumborder is shadow,
otherwise return 2.
c80bb5b63e