scripts/functions: always prefer the mirror

Currently, the mirror can be used either:
- as a fallback in case upstream is unavailable (default behavior)
- as the preferred source for downloads

But the most common use-case seems to provide a truely-LAN mirror
to speed up downloads in big corpos', and/or provide a 'trusted'
source for the tarballs.

So, make the following changes;
- if a mirror is specified, always try that before trying upstream

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.