fix weird all.bash failures by running deps.bash before make clean runs in pkg directory.

fix weird all.bash failures by running deps.bash before make clean runs in pkg directory.
before this change, if pkg/Make.deps is missing or broken, clean.bash fails and the build dies
but not until much later.

add freebsd to error message about valid values of $GOOS

TODO: would be nice if this process exited when an error occurred. subshells make it hard

R=rsc
CC=golang-dev
https://golang.org/cl/160065