nspawn: when resoliving UIDs/GIDs for "-u", do so in forked off /usr/bin/getent inste...

nspawn: when resoliving UIDs/GIDs for "-u", do so in forked off /usr/bin/getent instead of in-process

When the container runs a different native architecture than the host we
shouldn't attempt to load the container's NSS modules with the host's
libc. Instead, resolve UID/GID by invoking /usr/bin/getent in the
container. The tool should be fairly universally available and allows us
to do resolving of the UID/GID with the container's libc in a parsable
format.

https://bugs.freedesktop.org/show_bug.cgi?id=75733