netxml: added RW access, fixed FSD/shutdown duration bugs, etc.

 * Fixed bugs in resolution of FSD condition and computation of shutdown
duration.
 * Added System.* UPS variables.
 * Enabled RW access to appropriate UPS variables.
 * Added UPS veriables value convertors.
 * Added support for XML protocol v3 {GET|SET}_OBJECT query implementing getvar
and setvar routines.
 * netxml driver man page updated to include info about the driver-specific
configuration parameters.

Closes #59 (GitHub pull request: "Enhancement for netxml driver")

Pull request by: Frédéric BOHE <fredericbohe@eaton.com>