Bug#952421: Cannot use PAM modules which call subprocess e.g. pam_exec

[Ed Spiridonov]

The issue is fixed in this patch:
https://src.fedoraproject.org/rpms/vsftpd/raw/main/f/0026-Prevent-hanging-in-SIGCHLD-handler.patch

It applies flawlessly to the debian version of vsftpd.