Allow plugins to be written in C about oomd

[danobi]

This sounds like a useful idea and I think it should be possible. Similar to how there's a BaseKillPlugin, I could see a BaseCPlugin providing a C interface to oomd's plugin API.

In theory the only C++ interfaces you'd need a C API for are the ones inside BasePlugin.h. There's a bunch of helper functions scattered around util/ but it's not strictly necessary to use them in plugins.

Then you could have a plugins structured like:

$ ls
MyCPlugin.c
BaseCPlugin.cpp

$ head BaseCPlugin.cpp
#include "MyCPlugin.c"

REGISTER_PLUGIN(my_c_plugin, ...)
...

$ head MyCPlugin.c
// magic function BaseCPlugin.cpp will try to use
int init(CMonitoredResources *res, CPluginArgs *args) {
  ...
}

// magic function BaseCPlugin.cpp will try to use
CPluginRet run(COomdContext *ctx) {
  ...
}