pw: do not move /home/$user to /usr/home

When adding a user, pw will create the path to the home directory
if needed. However, if creating a path with just one component,
i.e. that appears to be in the root directory, pw would create the
directory in /usr, and create a symlink from the root directory.
Most commonly, this meant that the default of /home/$user would turn
into /usr/home/$user. This was added in a self-described kludge 26
years ago. It made (some) sense when root was generally a small
partition, with most of the space in /usr. However, the default is
now one large partition. /home really doesn't belong under /usr,
and anyone who wants to use /usr/home can specify it explicitly.
Remove the kludge to move /home under /usr and create the symlink,
and just use the specified path. Note that this operation was
done only on the first invocation for a path, and this happened most
commonly when adding a user during the install.

Modify the test that checked for the creation of the symlink to
verify that the symlink is *not* made, but rather a directory.
Add a test that intermediate directories are still created.

Reviewed by: rgrimes, bapt
Differential Revision: https://reviews.freebsd.org/D40085