pam_lastlog: Respect PAM_SILENT flag

pam_lastlog: Respect PAM_SILENT flag

pam_lastlog module will not log info about failed login if the session
was opened with PAM_SILENT flag.

Example use case enabled by this change:

    sudo --non-interactive program

If this command is run by another program expecting specific output from
the command run by sudo, the unexpected info about failed logins will
break this program.

* modules/pam_lastlog/pam_lastlog.c: Respect silent option.
  (_pam_session_parse): Unset LASTLOG_BTMP if PAM_SILENT is set.