kes Add plugin types in jcr.h to reduce need to cast.

kes  Add plugin types in jcr.h to reduce need to cast.
kes  Do better checking of plugin return values.
kes  Free plugin context after calling pluginFree() as
     requested by James.
kes  Allow plugin to call JobMessage with NULL context.
kes  Do not add plugin to global list if loadPlugin returns error.
kes  Rework plugin.h a bit to reduce dependence on Bacula.