setup-{osx,brew{,-system}}: restructure homebrew setup scripts

This should make it easier for me to populate a new homebrew directory in
a non-default path without screwing up my system wide homebrew install.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
setup-brew: install abduco, not dtach

Signed-off-by: Christopher Larson <kergoth@gmail.com>
setup-brew-system,setup-python: shift homebrew.pth into the former

Signed-off-by: Christopher Larson <kergoth@gmail.com>
setup-brew-system: obey HOMEBREW_PATH for shell setup

Signed-off-by: Christopher Larson <kergoth@gmail.com>
fish: set WORKON_HOME, obey it for pipsi

Signed-off-by: Christopher Larson <kergoth@gmail.com>