Kyua and ATF speak different naming styles. In this case, an
unprivileged user property can be named with underscore on the Kyua
side, and with a minus on the ATF side. Sometimes it is not obvious
which style should be used in which situation. For instance, a test case
may require this configuration property being set using require.config.
Also, a test case may want to read the property using something like
atf_tc_get_config_var(). Which names should be used in these cases?
From the perspective of the original code, it is expected to be this:

require.config unprivileged-user
atf_tc_get_config_var(tc, "unprivileged-user")

But, as long as Kyua is the main interface, its users expect to work
with kyua.conf(5), which says that it must be named as uprivileged_user
(with underscore). As a result, test authors tend to this instead:

require.config unprivileged_user
atf_tc_get_config_var(tc, "unprivileged_user")

Kyua already has hacks to understand both unprivileged_user and
unprivileged-user coming from require.config. And this patch covers the
missing second part -- make Kyua pass both names back to ATF as two
identical configuration properties named different ways.