Customisable elevations

First off, I fixed two small unrelated errors, these are
two separate commits: the first two commits.

The third commit lets the user add `elevation-day` and
`elevation-night` in redshift.conf to change the timespan
of the transitional period during twilight.
There is one limitation: `elevation-day` cannot be set to
a lower value than `elevation-night` to make the display
more red during the day than during the night.

Edit: I made fourth commit that makes sure the elevations
are not equal.