functions: add new helpers that create a dir and cd/pushd into it

A lot of places are currently doing:
mkdir -p foo/bar
cd foo/bar

Or even:
mkdir -p foo/bar
pushd foo/bar
[...]
popd

Provide both wrapper to ease doing this.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

This is NOT the official version of dialog.  This version has been

significantly modified from the original.  It is for use by the Linux

kernel configuration script.  Please do not bother Savio Lam with 

questions about this program.