system: replace nogroup with nobody

Currently, we define the so-called "overflow group" as 'nogroup'.

However, one practical issue is that systemd-sysusers will otherwise
create a 'nobody' group with gid 999, because that's is what is usual to
define the overflow group: users and groups are defined in LSB (Linux
Standard Base):

    https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/usernames.html

Quoting: "If the username exists on a system,then they should be in the
suggested corresponding group".

Only Debian and derivatives depart from this custom, naming it 'nogroup'
(hence the rationale for commit 908198e7 (system/skeleton: remove
spurious group 'nobody').

See also commit 9c67af2c (system/skeleton: use uid/gid 65534 for
nobody/nogroup), and a related discussion on LWN.net (key is "overflow
UID" which also applies to GID):

    https://lwn.net/Articles/695478/



Use the recommended groupname 'nobody'. Adapt packages accordingly.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
[yann.morin.1998@free.fr:
  - reword commit log
  - extend commit log with more references (commits and LWN)
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>