sys: linear_range: allow out-of-range values/windows

The existing linear_range API did not allow values or windows outside of
the linear range (returned -EINVAL). With this change values are allowed
outside of the range, being adjusted to the edge values (min/max)
instead. In the case of windows, it is allowed to have partial
intersection. In both cases, the API assigns a valid index (nearest) and
returns -ERANGE. This change is useful because the main client of the
linear range API, regulators, needs such behavior. For example, If an
application specifies a voltage range from 1.0V to 1.5V and the
regulator supports from 1.2V to 2.7V, the regulator can configure a
voltage that satisfies the condition: 1.2V.  With the current API, the
input would be refused because 1.0V lies outside of the 1.2V-2.7V range.

Also, for constant ranges, the minimum index is returned.

Tests have been updated/extended accordingly.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>