Change option --skip-proxy to --proxy. Rename getUsername to getUsernameFromSiteDB. Fixes #4308, #4309.

- Change option --skip-proxy to --proxy.
- Rename getHyperNewsName() function to getUsernameFromSiteDB() in CredentialInteractions.
- Rename getUseName() function to getUsername() in CredentialInteractions.
- Rename getUsername() function to getUsernameFromSiteDB() in client_utilities.
- Rename getUseName() function to getUsername() in client_utilities.
- New function getUserDN() in client_utilities.
- New wrapper functions getUsernameFromSiteDB_wrapped() and getUserDN_wrapped() in client_utilities, which respectively call getUsernameFromSiteDB() and getUserDN() catching exceptions and giving better error messages.
- New function getUserDNandUsernameFromSiteDB() in client_utilities, which calls the two wrapper functions above and returns the user DN and username in a dictionary.
- Use the new function getUserDNandUsernameFromSiteDB() in the checkusername command standalone method. 
- Use the new function getUserDNandUsernameFromSiteDB() in the checkwrite command, when the user did not specify the --lfn option, to retrieve the username. Before we were using getHyperNewsName() from  CredentialInteractions.
- When doing 'scram unsetenv -sh', do first 'which scram' to avoid error messages from scarm not being available.