Make pppd use blank username/password when explicitly specified

Make pppd use blank username/password when explicitly specified

Patch from Jon Dubovsky.

Previously pppd would use its default strategies for working out a
username and password/secret to use if the user gave the empty string
to the user and/or password options.  Now we set a flag when an
explicit username is given, and don't do the default username
calculation if the flag is set.  Similarly for the password.