mssqlclient.py commands and prompt improvements

[Mayfly277]

• Add -show argument to print command queries.
• Add some commands :

     enum_db                    - enum databases
     enum_links                 - enum linked servers
     enum_impersonate           - check logins that can be impersonate
     enum_logins                - enum login users
     enum_users                 - enum current db users
     enum_owner                 - enum db owner
     exec_as_user {user}        - impersonate with execute as user
     exec_as_login {login}      - impersonate with execute as login
     xp_dirtree {path}          - executes xp_dirtree on the path
     use_link {link}            - linked server to use (set use_link localhost to go back to local or use_link .. to get back one step)
     show_query                 - show query
     mask_query                 - mask query

• Improve the column size result
• Change the prompt indicating current login, current user, db in use and linkedserver if you use one.