Fix the issue fledge-iot#1069

In the 'plugin_developers_guide/03_south_C_plugins.html' documentation page,
the signature of some plugin functions involving PLUGIN_HANDLE is erroneous.

According to the definitions of the function pointers about the South
plugins ($FLEDGE_ROOT/C/services/south/include/south_plugin.h),
fix these signatures.

And use the appropriate C++ 'cast' operator,
more restrictive and accurate 'cast' operator than the C one,
for the 'PLUGIN_HANDLE' object.

Signed-off-by: Mikael Bourhis <mikael.bourhis@smile.fr>