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.
This is required to avoid include cycles, as some nxt_clone_* functions
depend on the credential structures, but nxt_process depends on clone
structures.
