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>

#!/bin/sh

set -e

printf "Running autoconf...\n"

autoconf -Wall --force

printf "Done. You may now run:\n    ./configure\n"