init: Use root's home directory in single-user mode

When starting single-user mode, use the home directory from user root as
current directory and for the HOME environment variable. If the
directory does not exist, set HOME=/.

Also adjust /root/.profile to stop setting HOME, since it should always
have been set now.

This is intended to keep shell startup files working in single-user mode
after /.profile has been removed.

Reviewed by: emaste, ivy, kevans
MFC after: 1 week
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D52527