Paramiko should ignore LOGNAME

[asomers]

Paramiko currently uses getpass.getuser() to determine the local username, which works by examining various environment variables, starting with LOGNAME. However, that can give the wrong answer following su. For example:

> sudo su -m nobody
su: /root/.bashrc: Permission denied
[nobody@myhost /mydirectory]$ echo $LOGNAME
root

This leads to authentication errors, at least when using hostbased authentication as in PR #2093, like this: paramiko.ssh_exception.AuthenticationException: /usr/libexec/ssh-keysign terminated with an error: not a valid request. I haven't yet figured out how to trigger the problem without hostbased authentication, but config.py and client.py both use getpass.getuser().

OpenSSH itself does not. It uses the process's uid. See https://github.com/freebsd/freebsd-src/blob/150486f6a940947fa7a7c6db357c3f92d07a955f/crypto/openssh/ssh.c#L660 . I suggest that Paramiko do the same. Doing that fixes the hostbased authentication errors after su. Do you agree? the Python code is

username = pwd.getpwuid(os.getuid()).pw_name