Rework pushd implementation.  Adds "pushd ++<n>" syntax.

Specifically, the old +n semantic is now ++n, while +n now just pulls a
single element out of the stack and moves it to the top.

For example, before

% dirs
a b c d e
% pushd
b a c d e
% pushd +2
c d e b a
%

whereas now

% dirs
a b c d e
% pushd
b a c d e
% pushd +2
c b a d e
% pushd ++2
a d e c b
%

There also is now a -V flag to pushd, which prints debugging info.