shell-common: canonicalize mingw

Newer versions are now including other things in the `uname -s`.  Let's
canonicalize it to just "mingw".
bash: only use tput on non-mingw platforms
shell-common: default to bolded colors for "light" versions under mingw

The shell doesn't interpret them. :-(
shell-common: use USERNAME if USER is empty

This helps to get the proper username in the prompt on Windows.