Add improved support for Eaton 5P

Add the necessary hooks to improve support for Eaton 5P range. This
includes post-processing of the model name, along with handling rules
for battery voltage (actual and nominal)
Closes #96: Install upsdrvctl to $prefix/sbin

Install upsdrvctl to $prefix/sbin rather than $driverexec. upsdrvctl has
been historically standing beside the drivers. It now resides in the
system binaries ($prefix/sbin) directory