Support PAM logins with email addresses
Add a call to `getpwnam()` in the PAM auth module, because the actual
username may be different from the requested username. For example SSSd
allows logging in with the email address, but we want the actual
username to be used throughout the application.
Fixes: #358
Signed-off-by: Aurélien Bompard <aurelien@bompard.org>