Add a login(true) option to the bash resource to invoke a shell with the target users environment

Users frequently want to be able to "run a script as a user" where the script will run the same as if they logged in as that user or su'd to it. In the current implementation, only the uid (and gid, if a group is given) are changed. So there should be a login(true) option to the bash resource. It needs to do the following (at least) to emulate being the logged in user:

• clear the PATH
• determine the user's HOME from the Etc module and set $HOME to that value
• Run bash as bash -l

This should cause the correct profile and rc files to be read, making the script run in a login-like environment.