fix a whole bunch of warning from shellcheck•••There are still quite some, this is a first chunk based on a given patch
applied where i felt confident enough.
Thanks: Herbert Parentes Fortes Neto <hpfn@ig.com.br> for the initial patch-set
Move arch specific handling to guess_*_vendor()•••Move arch specific handling from guess_vendor_arch_mirror() to
guess_dist_vendor() and rename functions accordingly. Introduce an
"ubuntu-ports" vendor -- should really be an "archive".