Compared to the previous implementation based on OpenSSL, the new implementation
has these advantages:
1. Strict and reliable detection of invalid strings, including strings with
less than 4 bytes of garbage at the end;
2. Allows to use Base64 strings without '=' padding.
The setuid/setgid syscalls requires root capabilities but if the kernel
supports unprivileged user namespace then the child process has the full
set of capabilities in the new namespace, then we can allow setting "user"
and "group" in such cases (this is a common security use case).
Tests were added to ensure user gets meaningful error messages for
uid/gid mapping misconfigurations.