sysusers: avoid creating spurious "nobody" group

On distros using Debian's base-passwd, the name of the group with gid 65534 is
nogroup. Currently, systemd-sysusers creates a spurious "nobody" group

  systemd-sysusers[243]: Creating group nobody with gid 996

That's both confusing and redundant, as the nobody user still has primary group
65534 aka nogroup, and the nobody group simply goes completely unused.

So explicitly specify the primary group of the nobody user, and add a line
ensuring that that group exists.

This is not a problem for Debian (or Ubuntu) itself, as they add their own
version of basic.conf in their systemd build logic. But it appears on for
example Yocto/OpenEmbedded.