net-analyzer/nagios-core: use default homedir/shell for "nagios" user.

net-analyzer/nagios-core: use default homedir/shell for "nagios" user.

With GLEP81 approved, we're finally going to be able to (properly)
share the "nagios" user between the packages that require it. As part
of the migration process, I'd like to clean up its home directory and
shell usage: I'm fairly sure that the default (/dev/null and
/sbin/nologin) will work, at least for nagios-core. Whether or not
this will affect any other packages is harder to discern.

I'm using this new ~arch version of nagios-core to test things out. In
this and later versions, we'll use the default "enewuser" arguments.
If this causes a problem, it should appear only in new installs
(enewuser is a no-op on upgrades), and with any luck someone will file
a bug to say it doesn't work.

Package-Manager: Portage-2.3.69, Repoman-2.3.16
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>