Stop generating broken .zshrc, .zshenv, and .bashrc files.

Previously, we'd end up embedding a newline into the line setting up
$ETC_HOME.  This would then cause the source to fail.  Also, the ~/
version of home was being referenced, but it was in quotes and doesn't
get expanded.  Let's just use $HOME instead, where possible.  .gitconfig
still needs the tilde version, since git doesn't do environment variable
expansion, but does do tilde expansion.