Sensitivity: Introduce mconfig_get_sensitivity and _set_

Sensitivity: Introduce mconfig_get_sensitivity and _set_

In a moment, we are going to make the in-use representation of the
sensitivity more complicated.  We will need accessor methods for use
by the configuration machinery.

Provide these accessor methods now, to reduce noise in the substantive
patch.  For now they just access the single double.

Rename the variable so that we catch all of the references.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>