Fix /root permissions after 'make installworld'

According to /etc/mtree/BSD.root.dist /root should have
0750 permissions, but the build target 'make installworld'
changes these to 0755.

This is caused by the installation of the configuration
files of sh(1) and csh(1).

Correct this by specifying the correct default /root permissions.

PR: 273342
Reviewed by: jilles
Approved by: jilles
MFC after: 2 weeks
Differential Revision:https://reviews.freebsd.org/D42395