Previously, the `-no-user-package` and `-no-global-package` flags
affected the "initial" stack only, while `user-package` and
`global-packages` appended to the end of the stack.

This commit changes the behavior of those flags, so that they are always
applied to the stack as a whole.

The effect of the GHC_PACKAGE_PATH environment variable has also been
changed: terminating it with a separator now adds the default package
dbs (user and global) instead of the initial stack.