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

call_to=@@DEFAULT_LD@@

case "${CTNG_LD_IS}" in

    bfd)    call_to=bfd;;

    gold)   call_to=gold;;

esac

exec "${0}.${call_to}" "$@"

exit $?