Fix indentation of \d footers for non-ASCII cases.

Multi-line "Inherits:" and "Child tables:" footers were misindented when
those strings' translations involved multibyte characters, because we were
using strlen() instead of an appropriate display width measurement.

In passing, avoid doing gettext() more than once per loop in these places.

While at it, fix pg_wcswidth(), which has been entirely broken since about
8.2, but fortunately has been unused for the same length of time.

Report and patch by Sergey Burladyan (bug #6480)