Introduce sanitize_str().

Currently we sanitize the status item strings for para_gui in
align_str() of gui.c. This implementation is flawed because it does
not work for wide character strings.

This patch provides an abstraction in string.c which works in both
the UFT-8 and non-UTF-8 case.

The flawed implementation of this function in gui.c is removed and
the surrounding code is changed to call the new function instead.